cmake SCALAPACK_LIBRARY_HAS_BLACS examples

dealii/dealii cmake/modules/FindDEAL_II_SCALAPACK.cmake :48 

CHECK_CXX_SOURCE_COMPILES(
    "
    #include <mpi.h>
    extern \"C\"
    {
      int Csys2blacs_handle(MPI_Comm comm);
    }
    int main() {
      const int res = Csys2blacs_handle(MPI_COMM_WORLD);
    }
    "
    SCALAPACK_LIBRARY_HAS_BLACS
  )