MPI_AINT_ADD and MPI_AINT_DIFF are functions and must be declared as
externals with the proper return type. This is already done properly
in the mpi and mpi_f08 modules; these declarations for these functions
were only missing from mpif.h (i.e., mpif-externals.h).
Thanks to Aboorva Devarajan (@AboorvaDevarajan) for the bug report.
Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
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