tests: ensure mdemo.at doesn't rely on installed libltdl.

tests: ensure mdemo.at doesn't rely on installed libltdl.

* tests/mdemo.at (configuer.ac): Build an included convenience
libltdl.
(Makefile.am): Add subdir-objects.
Remove override rules from legacy mdemo tests to reuse parent
libltdlc.la.
(LT_AT_CHECK_CONFIG): Add --with-included-ltdl to configure
arguments so that test cases always use the subproject libltdl
in preference to any installed libltdl.
Reported by Peter Rosin.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>