[cmake] fixes static build for macos with OpenMP enabled (fixes #6601)

[Mottl]

Fixes #6601 with -DBUILD_STATIC_LIB=ON on macos with USE_OPENMP enabled

[Mottl]

@microsoft-github-policy-service agree

[jameslamb]

Thanks very much for this, and sorry for breaking builds of the static library on macOS. We probably need to add some CI coverage of that.

Could you please instead skip this entire block, by adding a NOT BUILD_STATIC_LIB into this condition?

LightGBM/CMakeLists.txt

Line 755 in 9a76aae

if(APPLE AND USE_OPENMP)

All of this logic, like modifying RPATH entries, should be unnecessary for the static library.

[Mottl]

Thanks James. Done.

[jameslamb]

Thanks for the fix!

I pulled your branch onto my M2 mac and tested like this:

git remote add mottl [email protected]:Mottl/LightGBM.git
git fetch mottl macos_openmp_cmake_fix

cmake \
    -B build \
    -S . \
    -DBUILD_STATIC_LIB=ON \
    -DUSE_OPENMP=ON

cmake --build build --target all -j4

Saw everything build successfully, with 0 warnings or errors.

[ 89%] Built target lightgbm_objs
[ 97%] Linking CXX static library /Users/jlamb/repos/LightGBM/lib_lightgbm.a
[ 97%] Building CXX object CMakeFiles/lightgbm.dir/src/application/application.cpp.o
[ 97%] Building CXX object CMakeFiles/lightgbm.dir/src/main.cpp.o
[ 97%] Built target _lightgbm
[100%] Linking CXX executable /Users/jlamb/repos/LightGBM/lightgbm
[100%] Built target lightgbm

Also inspected the library a bit and everything looked correct.

ar -t ./lib_lightgbm.a
nm ./lib_lightgbm.a