libltdl uses LIBOBJS, which leak out into a parent project if both

libltdl uses LIBOBJS, which leak out into a parent project if both
share the same configure.ac.  Libtool itself doesn't compile any
code, so it is okay to do it during bootstrap, but other projects
need to treat libltdl as a configurable subproject to keep their
respective LIBOBJS separate:

* libltdl/configure.ac: New file, based on configure.ac.
* libltdl/Makefile.am (AC_LOCAL_AMFLAGS): Use our own m4
directory.
(ltdldata_DATA): List *all* the files that libtoolize --ltdl will
install.