OpenMPI is not correctly included, nested extern "C"

[Janus]

Nested 'extern "C"' breaks the build with C++ on GCC 7.1.

Also, if MED_HAVE_MPI is defined, MPI symbols are used even though it is not included.

Proposed fix for extern C:

in /usr/include/med.h:

around '#include "med_proto.h"' change to:

#ifdef __cplusplus
}
#endif
#include "med_proto.h"
#ifdef __cplusplus
extern "C" {
#endif

==============

Proposed fix for missing include: insert

#ifdef MED_HAVE_MPI
#include <mpi.h>
#endif

on line 24 in /usr/include/med_proto.h