gcc

$ gcc -fPIC -shared -c myctor.c -o myctor.o
$ gcc -fPIC -shared myctor.o -o libmyctor.so
$ LD_PRELOAD=$(pwd)/libmyctor.so ls         
myctor
libmyctor.so  myctor.c      myctor.o

$ % dis -t .init libmyctor.so
disassembly for libmyctor.so


section .init
_init()
    _init:      55                 pushl  %ebp
    _init+0x1:  89 e5              movl   %esp,%ebp
    _init+0x3:  83 e4 f0           andl   $0xfffffff0,%esp
    _init+0x6:  83 ec 0c           subl   $0xc,%esp
    _init+0x9:  53                 pushl  %ebx
    _init+0xa:  e8 00 00 00 00     call   +0x0	<_init+0xf>
    _init+0xf:  5b                 popl   %ebx
    _init+0x10: 81 c3 49 00 01 00  addl   $0x10049,%ebx
    _init+0x16: e8 35 ff ff ff     call   -0xcb	<frame_dummy>
    _init+0x1b: e8 a0 ff ff ff     call   -0x60	<__do_global_ctors_aux>
    _init+0x20: 5b                 popl   %ebx
    _init+0x21: c9                 leave  
    _init+0x22: c3                 ret 

libtool

$ libtool --mode=compile gcc -g -O -c myctor.c
libtool: compile:  gcc -g -O -c myctor.c  -fPIC -DPIC -o .libs/myctor.o
$ libtool --mode=link gcc -g -O -o libmyctor.la myctor.lo -rpath /usr/lib
libtool: link: rm -fr  .libs/libmyctor.a .libs/libmyctor.la
libtool: link: gcc -shared  -fPIC -DPIC -Wl,-z -Wl,text -Wl,-h -Wl,libmyctor.so.0 -o .libs/libmyctor.so.0.0.0  .libs/myctor.o    -O   -nostdlib -lc
libtool: link: (cd ".libs" && rm -f "libmyctor.so.0" && ln -s "libmyctor.so.0.0.0" "libmyctor.so.0")
libtool: link: (cd ".libs" && rm -f "libmyctor.so" && ln -s "libmyctor.so.0.0.0" "libmyctor.so")
libtool: link: ( cd ".libs" && rm -f "libmyctor.la" && ln -s "../libmyctor.la" "libmyctor.la" )

$ LD_PRELOAD=$(pwd)/.libs/libmyctor.so ls
libmyctor.la  myctor.c      myctor.lo     myctor.o

$ dis -t .init .libs/libmyctor.so
disassembly for .libs/libmyctor.so

dis: warning: failed to find section '.init' in '.libs/libmyctor.so'

myctor.c

#include <stdio.h>

void __attribute__((constructor)) myctor() {
    printf("myctor\n");
}