fix-ltdl: Fix LIBOBJ mangling to work with automake-1.16.

Apparently automake 1.16 changed how object files in a subdirectory are
named when using subdir-objects.  As the generated rules to copy compiled
objects to the ltdl_LTLIBOBJ substituted names, these rules are now failing
on current automake.

Fix this by explicitly setting a _SHORTNAME for the dummy libobj library,
which explicitly sets the name to use and thus it will be consistent
between different Automake versions.

Add new test cases to verify this behaviour.