Do not require an archive when the OMPI_MPIEXT_<ext>_HAVE_OBJECT
macro is defined to 0.
See `ompi/mpiext/example/configure.m4`.
Allow some extensions to be built on OS X since the creation of
archives with no files is not permitted.
Refs. open-mpi/ompi#6205
Signed-off-by: Gilles Gouaillardet <gilles@rist.or.jp>
Signed-off-by: KAWASHIMA Takahiro <t-kawashima@jp.fujitsu.com>
Signed-off-by: Jeff Squyres <jsquyres@cisco.com>