PMIx - override default paths at configure time if --with-pmix is used.

Previously the default paths continued to be tested even when new ones
were requested. This had as a consequence that if any of the new paths
was the same as any of the default ones (i.e. /usr or /usr/local), the
configure script was incorrectly erroring out specifying that a version
of PMIx was already found in a previous path.

Bug 5168.