message: Use the already exisisting macro MFEM_HAVE_GCC_PRAGMA_DIAGNOSTIC
diff: #ifdef MFEM_USE_GSLIB
  -#ifdef __GNUC__
  +#ifdef MFEM_HAVE_GCC_PRAGMA_DIAGNOSTIC
  // Ignore warnings from the gslib header (GCC version)
  #pragma GCC diagnostic ignored "-Wunused-function"
  -#endif // __GNUC__
  +#endif
  #include "gslib.h"
  