fix, descend into local libltdl/ subdir if --with-included-libltdl is passed on to configure

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

* bld/ltdl:
  fix, descend into local libltdl/ subdir if --with-included-libltdl is passed on to configure