This is a bit of overkill, but I'm cleaning out a bunch of other
libltdl-support assumptions, so I might as well do this one, too. The
test isn't built if we don't have libltdl support, but it had
half-hearted #if protection in it to make it safe to build even if we
didn't have libltdl support. This commit finishes that half-hearted
support.