libtool: Add option to reorder the shared library cache

libtool: Add option to reorder the shared library cache

Add option to reorder the shared library cache in OpenBSD so that user
preferred directories for shared libraries can be used when linking
before directories previously listed in the shared library cache.

This allows for users in OpenBSD to easily switch between versions of
libraries with the same name during testing.

* NEWS: Update for new (OpenBSD) option.
* build-aux/ltmain.in: Add option --reorder-cache=DIRS.
* doc/libtool.texi: Update documentation for new option.
* test/bug_71489.at: Alter test for OpenBSD to utilize new option.