libtool: Fix local testing of shared libraries on OpenBSD

libtool: Fix local testing of shared libraries on OpenBSD

On OpenBSD 7.5, the shared library cache is updated with finish_cmds,
which causes the test for 'Use local version' to fail. Simply, installed
versions of shared libraries take precedence over those locally built.

To ensure local versions of shared libraries can be tested properly and
more easily, additional options have been added to libtool to skip
executing finish_cmds, like 'ldconfig -m $libdir' in OpenBSD.

* build-aux/ltmain.in: Add libtool option, '--no-finish.
* doc/libtool.texi: Document new option in libtool.
* NEWS: Update.