Fix missing OpenMP support with Python 3.13

Author: pmachapman

.github/workflows/ci.yml

@@ -21,6 +21,9 @@ jobs:
 
     steps:
       - uses: actions/checkout@v5
+      - name: Install OpenMP on macOS
+        if: contains(matrix.os, 'macos')
+        run: brew install libomp
       - name: Set up Python ${{ matrix.python-version }}
         id: setup-python
         uses: actions/setup-python@v6
@@ -34,6 +37,16 @@ jobs:
           installer-parallel: true
       - name: Install dependencies
         run: poetry install --no-interaction --no-root
+        if: contains(matrix.os, 'macos')
+        env:
+          CPPFLAGS: -I/opt/homebrew/opt/libomp/include
+          LDFLAGS: -L/opt/homebrew/opt/libomp/lib
+          CFLAGS: -Xpreprocessor -fopenmp
+          CXXFLAGS: -Xpreprocessor -fopenmp
+          CMAKE_ARGS: >
+            -DOpenMP_C_FLAGS="-Xpreprocessor -fopenmp"
+            -DOpenMP_C_LIB_NAMES=omp
+            -DOpenMP_omp_LIBRARY=/opt/homebrew/opt/libomp/lib/libomp.dylib
       - name: Install machine
         run: poetry install --no-interaction
       - name: Lint with Black