While we require C99 to build Open MPI, we do not require C99 to build
user MPI applications. As such, we shouldn't have C99-style comments
(i.e., "//"-style) in mpi.h.in.
Thanks to @AdamSimpson for reporting the issue.
This commit simply converts a //-style comment to a /**/-style
comment. No code or logic changes.
Signed-off-by: Jeff Squyres <jsquyres@cisco.com>