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

This reverts commit b8a9f69c9d1fde6745b641925641b0a0dd77d61e.

I like my stuff hidden away in macro files, so move the changes
made in ca1d2e9b and 63ae0eef back into a macro called
SXE_CHECK_LIBLTDL.

Signed-off-by: Sebastian Freundt <freundt@fresse.org>

Conflicts:
	configure.ac
Merge branch 'bld/dist' into next

* bld/dist:
  (Partial) Revert "ltdl, move SXE_CHECK_LIBLTDL to configure.ac"