Allow for portability with Autoconf 2.59, Automake 1.9.6,

Allow for portability with Autoconf 2.59, Automake 1.9.6,
which lack AC_CONFIG_LIBOBJ_DIR support.

* bootstrap: Unless environment variable WORKING_LIBOBJS_SUPPORT
is set to true aka `:', symlink ..
* argz.c lt__dirent.c lt__strl.c: .. these from the copies in
libltdl.
* configure.ac (SUBDIR_LIBOBJS): Automake conditional, set if
bootstrap didn't make the extra copies of libobj sources in
top srcdir...
* Makefile.am: Add these to EXTRA_DIST if SUBDIR_OBJS is not set,
as well as the originals in libltdl/, in order to work when
bootstrapped both with old and newer Automake/Autoconf.