Fix dependencies using system-default compiler.

[nimble0]

Compiler arguments need to be passed to ExternalProjects otherwise they use the system default compiler.

[nimble0]

Might be a good idea to extend this to pass compile options as well.

[agruzdev]

Hello!

My initial idea was that CMAKE_TOOLCHAIN_FILE can be used to setup a custom compiler. Is it not convenient?

As for only CC/CXX args, isn't it easier to set them just as environment variables before running CMake?

[agruzdev]

Will it work on Windows?

[nimble0]

Env comes from CoreUtils in GnuWin32 which is supplied with several common developer packages such as Git Bash.

[nimble0]

I find it slightly easier, it's how cmake-gui changes compiler and it remembers the setting across runs (unlike environment variables).

But the other issue with it, is that it's a foot-gun as it is.

[TheNicker]

@agruzdev , Should dependencies added using add_subdirectory/ fetch_content so they'll be part of the build tree ?

[agruzdev]

@agruzdev , Should dependencies added using add_subdirectory/ fetch_content so they'll be part of the build tree ?

This was my initial version. That approach is super hard to maintain. Each add_subdirectory populates own variables, they might collide between dependencies. So CMake cache very quickly becomes a huge mess.
ExternalProjects lets to build and install each dependency isolated. Much easier to debug and maintain,

[TheNicker]

That approach is super hard to maintain

Fair.

In that case I like the idea, consider propagating additional variables (just an example)

include(ExternalProject)

set(_ep_cmake_args
  "-DCMAKE_RUNTIME_OUTPUT_DIRECTORY:PATH=${CMAKE_RUNTIME_OUTPUT_DIRECTORY}"
  "-DCMAKE_LIBRARY_OUTPUT_DIRECTORY:PATH=${CMAKE_LIBRARY_OUTPUT_DIRECTORY}"
  "-DCMAKE_ARCHIVE_OUTPUT_DIRECTORY:PATH=${CMAKE_ARCHIVE_OUTPUT_DIRECTORY}"
  "-DCMAKE_C_COMPILER:FILEPATH=${CMAKE_C_COMPILER}"
  "-DCMAKE_CXX_COMPILER:FILEPATH=${CMAKE_CXX_COMPILER}"
)

if(CMAKE_BUILD_TYPE)
  list(APPEND _ep_cmake_args
    "-DCMAKE_BUILD_TYPE:STRING=${CMAKE_BUILD_TYPE}"
  )
endif()

if(CMAKE_TOOLCHAIN_FILE)
  list(APPEND _ep_cmake_args
    "-DCMAKE_TOOLCHAIN_FILE:FILEPATH=${CMAKE_TOOLCHAIN_FILE}"
  )
endif()

if(CMAKE_MT)
  list(APPEND _ep_cmake_args
    "-DCMAKE_MT:FILEPATH=${CMAKE_MT}"
  )
endif()

if(CMAKE_RC_COMPILER)
  list(APPEND _ep_cmake_args
    "-DCMAKE_RC_COMPILER:FILEPATH=${CMAKE_RC_COMPILER}"
  )
endif()

ExternalProject_Add(my_external
  SOURCE_DIR "${some_source_dir}"
  BINARY_DIR "${CMAKE_BINARY_DIR}/_deps/my_external-build"
  CMAKE_ARGS ${_ep_cmake_args}
)