[compile error] Recent OpenMP not found on macOS

[themightyoarfish]

Describe the error

After brew install libomp which installs version 15.0.6, OpenMP is not found during the PCL install

To Reproduce

• Clone current master branch
• cmake .. -DCMAKE_PREFIX_PATH=$(brew --prefix libomp)

Result is

-- Could NOT find OpenMP_C (missing: OpenMP_C_FLAGS OpenMP_C_LIB_NAMES)
-- Could NOT find OpenMP_CXX (missing: OpenMP_CXX_FLAGS OpenMP_CXX_LIB_NAMES)
-- Could NOT find OpenMP (missing: OpenMP_C_FOUND OpenMP_CXX_FOUND C CXX)
-- Not found OpenMP

Your Environment (please complete the following information):

• PCL HEAD
• macOS 12.5.1
• Apple clang 14.0.0
• 3.24.2

If PCL was compiled from source or failure in compiling PCL itself:

• GPU, Kinfu, CUDA enabled? No
• List and Version of dependencies used Eigen 30960d485ec7e45b095d3ad206b2dbcc8bc835ba, Qhull 2020.2, qt 5.15.8, vtk 9.1.0
• Compilation flags are used

          cmake $(cmake_build_flags) .. \
              -DBUILD_CUDA=OFF \
              -DBUILD_GPU=OFF \
              -DBUILD_apps=ON \
              -DBUILD_apps_3d_rec_framework=OFF \
              -DBUILD_apps_cloud_composer=OFF \
              -DBUILD_apps_in_hand_scanner=OFF \
              -DBUILD_apps_modeler=OFF \
              -DBUILD_apps_point_cloud_editor=ON \
              -DBUILD_examples=OFF \
              -DBUILD_features=ON \
              -DBUILD_filters=ON \
              -DBUILD_io=ON \
              -DBUILD_kdtree=ON \
              -DBUILD_keypoints=ON \
              -DBUILD_ml=ON \
              -DBUILD_ml=ON \
              -DBUILD_octree=ON \
              -DBUILD_outofcore=OFF \
              -DBUILD_people=OFF \
              -DBUILD_recognition=ON \
              -DBUILD_registration=ON \
              -DBUILD_segmentation=ON \
              -DBUILD_stereo=ON \
              -DBUILD_surface=ON \
              -DBUILD_tools=ON \
              -DBUILD_tools=ON \
              -DBUILD_tracking=ON \
              -DBUILD_visualization=ON \
              -DBoost_USE_DEBUG_RUNTIME=OFF \
              -DWITH_CUDA=OFF \
              -DWITH_VTK=ON \
              -DCMAKE_BUILD_TYPE=Release

Possible Solution

I run into OpenMP issues with different libraries all the time, I am suspecting that cmake just doesn't handle this correctly on recent macOSes, but I'm not sure.

[themightyoarfish]

Actually it seems that this isn't even a PCL problem:

$ brew --prefix libomp
/opt/homebrew/opt/libomp
$ cat ../CMakeLists.txt
project(openmp-bug)
list(APPEND CMAKE_PREFIX_PATH /opt/homebrew/opt/libomp/)
find_package(OpenMP REQUIRED)
$ cmake .. 
CMake Error at /opt/homebrew/Cellar/cmake/3.24.2/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
  Could NOT find OpenMP_C (missing: OpenMP_C_FLAGS OpenMP_C_LIB_NAMES)
Call Stack (most recent call first):
  /opt/homebrew/Cellar/cmake/3.24.2/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:594 (_FPHSA_FAILURE_MESSAGE)
  /opt/homebrew/Cellar/cmake/3.24.2/share/cmake/Modules/FindOpenMP.cmake:545 (find_package_handle_standard_args)
  CMakeLists.txt:3 (find_package)

So something seems wholly broken on macos cmake.