External pmix installs are frequently in non-standard locations and
the path to their shared libraries are not ldconfig'd in because
there may be multiple pmix installs.
The way the configury was set up prior to this patch, the configuration
would fail soon after the PMIX config stuff was called because it
added some pmix lib stuff to the LDFLAGS, resulting in configury tests
for things that require running the configure test to fail.
This patch avoids this problem by resetting the LDFLAGS and LIBS back
to what they were prior to the run of the external PMIX detection.
The CFLAGS setting is left because there are many places in the ompi
and opal source code where pmix_common.h needs to be included.
Signed-off-by: Howard Pritchard <hppritcha@gmail.com>