* m4/ltdl.m4 (AC_WITH_LTDL): Remove redundant AC_REQUIRE calls.

* m4/ltdl.m4 (AC_WITH_LTDL): Remove redundant AC_REQUIRE calls.
Either set variables to allow a client to link with the installed
libltdl, or else compile a local libltdl as a convenience library.
* libtoolize.in (opt_ltdl): Removed in favour of...
(ltdldir): ...this, which is defaulted to "libltdl" or can be set
by the user with an argument to --ltdl.
(Main): Use it!
* doc/libtool.texi (Distributing libltdl): Improved documentation
on libltdl.
(Invoking libtoolize): Document new optional --ltdl target
directory argument.