CI: Disable building with OpenMP on Apple Silicon

Building with OpenMP causes some test failures for macOS on Apple
Silicon, see:
ElmerCSC#634 (comment)
The affected tests currently are: `H1BasisEvaluation` and
`SD_H1BasisEvaluation`.

For the time being, disable building with OpenMP on Apple Silicon in CI.

OpenMP is needed as a transitional build dependency of the SuiteSparse
package from Homebrew. So, also disable building with CHOLMOD.

Note that the tests with OpenMP fail for optimization level `-O3` but
succeed with `-O2`.

.github/workflows/build-macos-homebrew.yaml

@@ -18,12 +18,28 @@ jobs:
 
     runs-on: ${{ matrix.os }}
 
+    name: ${{ matrix.os }} (${{ matrix.openmp }} OpenMP)
+
     strategy:
       # Allow other runners in the matrix to continue if some fail
       fail-fast: false
 
       matrix:
-        os: [macos-14, macos-13]
+        os: [macos-14]
+        openmp: [without]
+        # Building with OpenMP causes some test failures on macOS on Apple
+        # Silicon.
+        # The affected tests currently are: H1BasisEvaluation and
+        # SD_H1BasisEvaluation.
+        # For the time being, disable building with OpenMP in CI on Apple
+        # Silicon.
+        # OpenMP is also needed as a transitional build dependency of the
+        # SuiteSparse package from Homebrew.
+        # FIXME: Consider building with OpenMP again when this (potentially
+        # upstream) issue has been fixed.
+        include:
+          - os: macos-13
+            openmp: with
 
     steps:
       - name: get CPU information
@@ -49,13 +65,14 @@ jobs:
           brew install --overwrite python@3.12 python@3.13
           brew reinstall gcc
           brew install \
-            cmake libomp openblas open-mpi suitesparse \
+            cmake openblas open-mpi \
+            ${{ matrix.openmp == 'with' && 'libomp suitesparse' || '' }} \
             qwt vtk opencascade
           echo "HOMEBREW_PREFIX=$(brew --prefix)" >> $GITHUB_ENV
 
       - name: configure
         env:
-          LDFLAGS: -L${{ env.HOMEBREW_PREFIX }}/opt/libomp/lib -lomp
+          LDFLAGS: ${{ matrix.openmp == 'with' && '-L${{ env.HOMEBREW_PREFIX }}/opt/libomp/lib -lomp' || '' }}
         run: |
           mkdir ${GITHUB_WORKSPACE}/build
           cd ${GITHUB_WORKSPACE}/build
@@ -66,17 +83,19 @@ jobs:
             -DCMAKE_Fortran_COMPILER=gfortran \
             -DCMAKE_INSTALL_PREFIX="${GITHUB_WORKSPACE}/usr" \
             -DBLA_VENDOR="OpenBLAS" \
-            -DCMAKE_PREFIX_PATH="${HOMEBREW_PREFIX}/opt/libomp;${HOMEBREW_PREFIX}/opt/openblas;${HOMEBREW_PREFIX}/opt/qt;${HOMEBREW_PREFIX}/opt/qwt" \
-            -DWITH_OpenMP=ON \
-            -DOpenMP_C_FLAGS="-Xclang -fopenmp -I${HOMEBREW_PREFIX}/opt/libomp/include" \
-            -DOpenMP_CXX_FLAGS="-Xclang -fopenmp -I${HOMEBREW_PREFIX}/opt/libomp/include" \
-            -DOpenMP_Fortran_FLAGS="-fopenmp -I${HOMEBREW_PREFIX}/opt/libomp/include" \
+            -DCMAKE_PREFIX_PATH="$( [ "${{ matrix.openmp }}" == "with" ] && echo "${HOMEBREW_PREFIX}/opt/libomp;")${HOMEBREW_PREFIX}/opt/openblas;${HOMEBREW_PREFIX}/opt/qt;${HOMEBREW_PREFIX}/opt/qwt" \
+            ${{ matrix.openmp == 'with' \
+              && '-DWITH_OpenMP=ON \
+                  -DOpenMP_C_FLAGS="-Xclang -fopenmp -I${HOMEBREW_PREFIX}/opt/libomp/include" \
+                  -DOpenMP_CXX_FLAGS="-Xclang -fopenmp -I${HOMEBREW_PREFIX}/opt/libomp/include" \
+                  -DOpenMP_Fortran_FLAGS="-fopenmp -I${HOMEBREW_PREFIX}/opt/libomp/include"' \
+              || '-DWITH_OpenMP=OFF' }} \
             -DWITH_LUA=ON \
             -DWITH_MPI=ON \
             -DMPI_TEST_MAXPROC=2 \
             -DWITH_Zoltan=OFF \
             -DWITH_Mumps=OFF \
-            -DWITH_CHOLMOD=ON \
+            $([ "${{ matrix.openmp }}" == "with" ] && echo "-DWITH_CHOLMOD=ON" || echo "-DWITH_CHOLMOD=OFF") \
             -DWITH_ElmerIce=ON \
             -DWITH_ELMERGUI=ON \
             -DWITH_QT6=ON \