Using ExternalProject with cmake

CMakeLists.txt

cmake_minimum_required(VERSION 3.10)
include(ExternalProject)
ExternalProject_Add(fmtlib
  GIT_REPOSITORY https://github.com/fmtlib/fmt.git
  EXCLUDE_FROM_ALL TRUE
  BUILD_COMMAND $(MAKE) fmt
  STEP_TARGETS build)
set(fmtlib_BINARY_DIR "${CMAKE_BINARY_DIR}/fmtlib-prefix/src/fmtlib-build")
set(fmtlib_SOURCE_DIR "${CMAKE_BINARY_DIR}/fmtlib-prefix/src/fmtlib/include")
add_executable(fmttest fmttest.cpp)
add_dependencies(fmttest fmtlib-build)
target_link_libraries(fmttest ${fmtlib_BINARY_DIR}/libfmt.a)
target_include_directories(fmttest PUBLIC ${fmtlib_SOURCE_DIR})

# Tested with fmttest.cpp:
#
#   #include "fmt/format.h"
#   int main() { fmt::print("Hello {}!\n", "world"); }
#
# The trouble is, there is no easy way to make network-update optional.
# It kicks and screams if there is no network connection. There has to be a way
# To make updates optional.

maybe have a look at how this is done: http://google.github.io/googletest/quickstart-cmake.html