When compiling mpi.h with a modern C++ compiler and a high degree of
pickyness (e.g., -Wold-style-cast), casting using (void*) in the
OMPI_PREDEFINED_GLOBAL and MPI_STATUS*_IGNORE macros will emit
warnings. So if we're compiling with a C++ compiler, use C++'s
static_cast<> instead of (void*).
Thanks to @shadow-fax for identifying the issue.
Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
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