CMake compilation as a subdirectory

[joguenzl]

Hello everyone,

when compiling trexio with CMake as a subdirectory of a larger project, e.g. with the following structure:

Project/
  src/
  external/
    trexio/

including trexio in the root CMakeLists.txt as add_subdirectory(external/trexio) has CMAKE_SOURCE_DIR pointing to Project instead of Project/external/trexio which breaks functionality. It is fixed by using PROJECT_SOURCE_DIR instead, which is identical to CMAKE_SOURCE_DIR if trexio is installed on its own.

Furthermore, I made FindTREXIO.cmake check whether TREXIO_DIR is set as a CMake variable before loading it from the environment variable.

[q-posev]

Thanks a lot @joguenzl, looks good to me! Just need to confirm one thing before merging.

@wavefunction91 - this is partially related to what we have discussed about making TREXIO compatible with FetchContent functionality. I believe you would need to substitute CMAKE_SOURCE_DIR with PROJECT_SOURCE_DIR the way @joguenzl did in this PR. Is there no conflict with the work you were planning to do for FetchContent?

[q-posev]

MacOS CI is broken because I need to bump the min numpy version requirements...

[q-posev]

I think we can merge as is.

[wavefunction91]

@q-posev ah, sorry. Didn't see the original ping. Yes, this was one of the changes we had to make, exactly in the way we made it. No conflict at all!

[q-posev]

@wavefunction91 no problem, thanks for the update!