configury: remove vestigial libltdl references.

M4 no longer uses libltdl.
* configure.ac: Call LT_LIB_DLLOAD to set LIBADD_DL correctly.
* Makefile.am (m4_libm4_la_LIBADD): Remove LIBLTDL.
(m4_libm4_la_DEPENDENCIES): Remove LTDLDEPS.
Reported by Pavel Raiskup

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