config.toml

[target.x86_64-pc-windows-gnu]
linker = "x86_64-w64-mingw32-gcc"

[target.i686-pc-windows-gnu]
linker = "i686-w64-mingw32-gcc"
rustflags = "-C panic=abort"

libtor-windows-i686.log

$ RUST_BACKTRACE=1 cargo build --target i686-pc-windows-gnu --features vendored-openssl
   Compiling libtor-sys v43.6.0+0.4.3.6 (/mnt/c/Users/justin/dev/libtor-sys)
error: failed to run custom build command for `libtor-sys v43.6.0+0.4.3.6 (/mnt/c/Users/justin/dev/libtor-sys)`

Caused by:
  process didn't exit successfully: `/mnt/c/Users/justin/dev/libtor-sys/target/debug/build/libtor-sys-f519f91522c41b80/build-script-build` (exit code: 101)
--- stdout
OPT_LEVEL = Some("0")
CC_i686-pc-windows-gnu = None
CC_i686_pc_windows_gnu = None
TARGET_CC = None
CC = None
CROSS_COMPILE = None
CFLAGS_i686-pc-windows-gnu = None
CFLAGS_i686_pc_windows_gnu = None
TARGET_CFLAGS = None
CFLAGS = None
CRATE_CC_NO_DEFAULTS = None
DEBUG = Some("true")
CARGO_CFG_TARGET_FEATURE = Some("fxsr,sse,sse2")
running: "/mnt/c/Users/justin/dev/libtor-sys/target/i686-pc-windows-gnu/debug/build/libtor-sys-1e48e575498d39ad/out/libevent/libevent-src/configure" "--host=i686-pc-windows-gnu" "--prefix=/mnt/c/Users/justin/dev/libtor-sys/target/i686-pc-windows-gnu/debug/build/libtor-sys-1e48e575498d39ad/out/libevent" "--disable-shared" "--enable-static" "--with-pic" "--disable-samples" "--disable-openssl" "--disable-libevent-regress" "--disable-debug-mode" "--disable-dependency-tracking"
checking for a BSD-compatible install... /usr/bin/install -c
checking whether build environment is sane... yes
checking for i686-pc-windows-gnu-strip... no
checking for strip... strip
checking for a thread-safe mkdir -p... /usr/bin/mkdir -p
checking for gawk... gawk
checking whether make sets $(MAKE)... yes
checking whether make supports nested variables... yes
checking whether make supports nested variables... (cached) yes
checking whether make supports the include directive... yes (GNU style)
checking for i686-pc-windows-gnu-gcc... i686-w64-mingw32-gcc
checking whether the C compiler works... yes
checking for C compiler default output file name... a.exe
checking for suffix of executables... .exe
checking whether we are cross compiling... yes
checking for suffix of object files... o
checking whether we are using the GNU C compiler... yes
checking whether i686-w64-mingw32-gcc accepts -g... yes
checking for i686-w64-mingw32-gcc option to accept ISO C89... none needed
checking whether i686-w64-mingw32-gcc understands -c and -o together... yes
checking dependency style of i686-w64-mingw32-gcc... none
checking how to run the C preprocessor... i686-w64-mingw32-gcc -E
checking for grep that handles long lines and -e... /usr/bin/grep
checking for egrep... /usr/bin/grep -E
checking for ANSI C header files... yes
checking for sys/types.h... yes
checking for sys/stat.h... yes
checking for stdlib.h... yes
checking for string.h... yes
checking for memory.h... yes
checking for strings.h... yes
checking for inttypes.h... yes
checking for stdint.h... yes
checking for unistd.h... yes
checking minix/config.h usability... no
checking minix/config.h presence... no
checking for minix/config.h... no
checking whether it is safe to define __EXTENSIONS__... yes
checking build system type... x86_64-pc-linux-gnu
checking host system type... 
--- stderr
Invalid configuration `i686-pc-windows-gnu': machine `i686-pc-windows' not recognized
configure: error: /bin/bash /mnt/c/Users/justin/dev/libtor-sys/target/i686-pc-windows-gnu/debug/build/libtor-sys-1e48e575498d39ad/out/libevent/libevent-src/config.sub i686-pc-windows-gnu failed
thread 'main' panicked at '
command did not execute successfully, got: exit code: 1

build script failed, must exit now', /home/justin/.cargo/registry/src/github.com-1ecc6299db9ec823/autotools-0.2.1/src/lib.rs:449:5
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1069
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1504
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:218
  10: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:511
  11: rust_begin_unwind
             at src/libstd/panicking.rs:419
  12: std::panicking::begin_panic_fmt
             at src/libstd/panicking.rs:373
  13: autotools::fail
             at /home/justin/.cargo/registry/src/github.com-1ecc6299db9ec823/autotools-0.2.1/src/lib.rs:449
  14: autotools::run
             at /home/justin/.cargo/registry/src/github.com-1ecc6299db9ec823/autotools-0.2.1/src/lib.rs:437
  15: autotools::Config::build
             at /home/justin/.cargo/registry/src/github.com-1ecc6299db9ec823/autotools-0.2.1/src/lib.rs:373
  16: build_script_build::build_libevent
             at ./build.rs:86
  17: build_script_build::main
             at ./build.rs:339
  18: std::rt::lang_start::{{closure}}
             at /home/justin/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:67
  19: std::rt::lang_start_internal::{{closure}}
             at src/libstd/rt.rs:52
  20: std::panicking::try::do_call
             at src/libstd/panicking.rs:331
  21: std::panicking::try
             at src/libstd/panicking.rs:274
  22: std::panic::catch_unwind
             at src/libstd/panic.rs:394
  23: std::rt::lang_start_internal
             at src/libstd/rt.rs:51
  24: std::rt::lang_start
             at /home/justin/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:67
  25: main
  26: __libc_start_main
  27: _start
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

libtor-windows-x86.log

$ cargo build --target x86_64-pc-windows-gnu
   Compiling cc v1.0.58
   Compiling pkg-config v0.3.18
   Compiling autocfg v1.0.0
   Compiling libc v0.2.73
   Compiling fs_extra v1.1.0
   Compiling openssl-src v111.10.2+1.1.1g
   Compiling autotools v0.2.1
   Compiling libz-sys v1.0.25
   Compiling openssl-sys v0.9.58
   Compiling libtor-sys v43.6.0+0.4.3.6 (/mnt/c/Users/justin/dev/libtor-sys)
error: failed to run custom build command for `libtor-sys v43.6.0+0.4.3.6 (/mnt/c/Users/justin/dev/libtor-sys)`

Caused by:
  process didn't exit successfully: `/mnt/c/Users/justin/dev/libtor-sys/target/debug/build/libtor-sys-06bf74317166871d/build-script-build` (exit code: 101)
--- stdout
OPT_LEVEL = Some("0")
CC_x86_64-pc-windows-gnu = None
CC_x86_64_pc_windows_gnu = None
TARGET_CC = None
CC = None
CROSS_COMPILE = None
CFLAGS_x86_64-pc-windows-gnu = None
CFLAGS_x86_64_pc_windows_gnu = None
TARGET_CFLAGS = None
CFLAGS = None
CRATE_CC_NO_DEFAULTS = None
DEBUG = Some("true")
CARGO_CFG_TARGET_FEATURE = Some("fxsr,sse,sse2")
running: "/mnt/c/Users/justin/dev/libtor-sys/target/x86_64-pc-windows-gnu/debug/build/libtor-sys-a35bdbf51f658f32/out/libevent/libevent-src/configure" "--host=x86_64-pc-windows-gnu" "--prefix=/mnt/c/Users/justin/dev/libtor-sys/target/x86_64-pc-windows-gnu/debug/build/libtor-sys-a35bdbf51f658f32/out/libevent" "--disable-shared" "--enable-static" "--with-pic" "--disable-samples" "--disable-openssl" "--disable-libevent-regress" "--disable-debug-mode" "--disable-dependency-tracking"
checking for a BSD-compatible install... /usr/bin/install -c
checking whether build environment is sane... yes
checking for x86_64-pc-windows-gnu-strip... no
checking for strip... strip
checking for a thread-safe mkdir -p... /usr/bin/mkdir -p
checking for gawk... gawk
checking whether make sets $(MAKE)... yes
checking whether make supports nested variables... yes
checking whether make supports nested variables... (cached) yes
checking whether make supports the include directive... yes (GNU style)
checking for x86_64-pc-windows-gnu-gcc... x86_64-w64-mingw32-gcc
checking whether the C compiler works... yes
checking for C compiler default output file name... a.exe
checking for suffix of executables... .exe
checking whether we are cross compiling... no
checking for suffix of object files... o
checking whether we are using the GNU C compiler... yes
checking whether x86_64-w64-mingw32-gcc accepts -g... yes
checking for x86_64-w64-mingw32-gcc option to accept ISO C89... none needed
checking whether x86_64-w64-mingw32-gcc understands -c and -o together... yes
checking dependency style of x86_64-w64-mingw32-gcc... none
checking how to run the C preprocessor... x86_64-w64-mingw32-gcc -E
checking for grep that handles long lines and -e... /usr/bin/grep
checking for egrep... /usr/bin/grep -E
checking for ANSI C header files... yes
checking for sys/types.h... yes
checking for sys/stat.h... yes
checking for stdlib.h... yes
checking for string.h... yes
checking for memory.h... yes
checking for strings.h... yes
checking for inttypes.h... yes
checking for stdint.h... yes
checking for unistd.h... yes
checking minix/config.h usability... no
checking minix/config.h presence... no
checking for minix/config.h... no
checking whether it is safe to define __EXTENSIONS__... yes
checking build system type... x86_64-pc-linux-gnu
checking host system type... 
--- stderr
Invalid configuration `x86_64-pc-windows-gnu': machine `x86_64-pc-windows' not recognized
configure: error: /bin/bash /mnt/c/Users/justin/dev/libtor-sys/target/x86_64-pc-windows-gnu/debug/build/libtor-sys-a35bdbf51f658f32/out/libevent/libevent-src/config.sub x86_64-pc-windows-gnu failed
thread 'main' panicked at '
command did not execute successfully, got: exit code: 1

build script failed, must exit now', /home/justin/.cargo/registry/src/github.com-1ecc6299db9ec823/autotools-0.2.1/src/lib.rs:449:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I was able to build a hello world project on x86_64 but not i686: https://github.com/justinmoon/cross-compile