ci(macos): use omp enabled homebrew-llvm-clang

Ref
- https://github.com/Rdatatable/data.table/wiki/Installation/370c4629742886190c08de5e199a8ca8d3d86b24
- https://github.com/actions/runner-images/blob/5c04ad10e394b07619bd68a62b3cd7665e7a3cb3/images/macos/macos-14-arm64-Readme.md

.github/workflows/tests.yml

@@ -38,6 +38,12 @@ jobs:
         if: ${{ runner.os != 'Windows' }}
       - name: Install dependencies
         run: |
+          if [ "$RUNNER_OS" == "macOS" ]; then
+              export CC="$(brew --prefix llvm@15)/bin/clang"
+              export CXX="$(brew --prefix llvm@15)/bin/clang++"
+              CPPFLAGS = "-I$(brew --prefix libomp)/include"
+              LDFLAGS = "-L$(brew --prefix libomp)/lib"
+          fi
           conda info
           conda install -c conda-forge numpy cython h5py
           python -m pip install --upgrade pip setuptools wheel

setup.py

@@ -128,7 +128,15 @@ def _get_openmp_flags():
     elif sys.platform == 'darwin':
         if (os.environ.get('CC') is not None and
            os.environ.get('CXX') is not None):
-            return ['-fopenmp'], ['-fopenmp']
+           cppflags = os.environ.get('CPPFLAGS')
+           ldflags = os.environ.get('LDFLAGS')
+           if cppflags is not None ldflags is not None:
+                print('*'*80)
+                print(cppflags.split(", "))
+                print(ldflags.split(", "))
+                return cppflags.split(", "), ldflags.split(", ")
+            else:
+                return ['-fopenmp'], ['-fopenmp']
         else:
             return ['-Xpreprocessor', '-fopenmp'], ['-lomp']
     else: