Add Mac OS OpenMP flags

Author: ruven

.github/workflows/c-cpp.yml

@@ -37,7 +37,7 @@ jobs:
       run: |
         ./autogen.sh
         export CFLAGS="-I/opt/homebrew/include -I/opt/homebrew/opt/libomp/include"
-        export CPPFLAGS="${CFLAGS}"
+        export CPPFLAGS="${CFLAGS} -Xpreprocessor -fopenmp"
         export LDFLAGS="-L/opt/homebrew/lib -L/opt/homebrew/opt/libomp/lib"
         export LIBS="-lomp"
         ./configure