chore, fix, actually descend into libltdl

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

* bld/ltdl:
  chore, fix, actually descend into libltdl
  chore, put at least half the ugly bits to ltdl initialisation into SXE_CHECK_LIBLTDL
  ltdl, workaround libtool LT_CONFIG_H bug
  ltdl, use LTDL_INIT's "recursive" mode
  ltdl, move SXE_CHECK_LIBLTDL to configure.ac
  ltdl, remove unsusd check for ltdl.h