Getting the value of ltdldatafiles from a submake was not

Getting the value of ltdldatafiles from a submake was not
portable, and had problems with GNU make-3.80 at least.  This
changeset pulls the installation rules for libtoolize's copy of
the libltdl sources out of libltdl/Makefile.am so that
ltdldatafiles is defined in the same Makefile that uses it.  Also,
this makes a start at cleaning up libltdl/Makefile.am so that it
is useful in a package that uses `libtoolize --ltdl':

* libltdl/loaders/Makefile.am: Removed entirely.
* libltdl/Makefile.am (dlopen.la, dld_link.la, dyld.la)
(load_add_on.la, loadlibrary.la, shl_load.la): All built from here
now that the loaders submake has gone.
(show-ltdldatafiles): Removed.  No longer required.
(ltdldatadir, ltdldatafiles, install-local): Moved from here...
* Makefile.am (ltdldatadir, ltdldatafiles, install-local): ..to
here.
(libtoolize): Substitute $(ltdldatafiles) directly without calling
make to get the value from libltdl/Makefile.