Fix library search order in our libtool.

Fix library search order in our libtool.

Before, libtool first searched all directories for .la files, and if
search failed, switched to actual libraries (.so/.a). But the correct way
is to check each directory first for .la, then for .so/.a.

The problem was reported by ajacoutot@ who verified that the patch fixes
his case and okay'ed the initial diff. And jasper@'s bulk uncovered a bug
which is fixed now (verified by another bulk).