opal_search_libs.m4: improvements from Dave Goodell's review

Unfortunately, I didn't see Dave's review until CMR #4841 went in.
Oops.

This commit is a handful of minor m4 improvements to that original
r32447 commit.  It can go to v1.8.3, just to minimize drift between
trunk and v1.8.

Reviewed by Dave Goodell.

cmr=v1.8.3:reviewer=ompi-rm1.8
Remove a lost #include.