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.