ltdl, remove unsusd check for ltdl.h
ltdl, move SXE_CHECK_LIBLTDL to configure.ac

autoreconf likes to see this in configure.ac. Most important It failed to
recongnice the LTDL_INIT mode, which is needed to guess the right
libtoolize options.
ltdl, use LTDL_INIT's "recursive" mode

This decreases the distdir size about 1.3 MB and increases configure
speed.
ltdl, workaround libtool LT_CONFIG_H bug

Building embedded libltdl fails because config.h is located in
subdir src. See libtool bug #12262
http://lists.gnu.org/archive/html/bug-libtool/2012-08/msg00007.html