Revert "(Partial) Revert "ltdl, move SXE_CHECK_LIBLTDL to configure.ac""

This reverts commit 6a0e671472594aa5ccba0a4ebdfd841a70eab7ef.

PEBKAC.
chore, put at least half the ugly bits to ltdl initialisation into SXE_CHECK_LIBLTDL

Older libtools want LTDL_INIT and LT_CONFIG_LTDL_DIR in c.ac.
See https://github.com/hroptatyr/uterus/pull/1#issuecomment-16110043

Signed-off-by: Sebastian Freundt <freundt@fresse.org>
Merge branch 'bld/dist' into next

* bld/dist:
  chore, put at least half the ugly bits to ltdl initialisation into SXE_CHECK_LIBLTDL
  Revert "(Partial) Revert "ltdl, move SXE_CHECK_LIBLTDL to configure.ac""
Merge branch 'next'

* next:
  chore, put at least half the ugly bits to ltdl initialisation into SXE_CHECK_LIBLTDL
  Revert "(Partial) Revert "ltdl, move SXE_CHECK_LIBLTDL to configure.ac""