Workaround for compiler error with OpenMPI

See open-mpi/ompi#5157 (comment)
for an explanation of this "hack that you can do that's gross but safe".

I agree that it's gross but they've at least got their workaround macro
nicely namespaced so it shouldn't interact with other code or other MPI
implementations.

Without some workaround, including OpenMPI's mpi.h in C++ code is a
compiler warning on many compilers and a compiler *error* on others,
including the last couple g++ releases.  I can't build nekRS without
this.

Author: roystgnr

src/core/nrssys.hpp

@@ -57,6 +57,9 @@
 #define dlongString "long long int"
 #endif
 
+// Workaround for https://github.com/open-mpi/ompi/issues/5157
+#define OMPI_SKIP_MPICXX 1
+
 #include <mpi.h>
 #include "occa.hpp"
 #include "ogs.hpp"