Move libltdl build mode options from LT_CONFIG_LTDL_DIR to

Move libltdl build mode options from LT_CONFIG_LTDL_DIR to
LTDL_INIT.  Accept (optional) new 'convenience' and 'installable'
options in lieu of LTDL_CONVENIENCE and LTDL_INSTALL macros. And
also, implement --with-included-ltdl, --with-ltdl-lib and
--with-ltdl-include configure-time options:

* libltdl/m4/ltoptions.m4: Associate existing option settings with
LT_INIT.
(LT_OPTION_DEFINE): Associate options 'nonrecursive', 'recursive',
'subproject', 'installable' and 'convenience' with LTDL_INIT.
(_LT_MANGLE_OPTION, _LT_SET_OPTION, _LT_IF_OPTION)
(_LT_UNLESS_OPTIONS, _LT_SET_OPTIONS): Add MACRO-NAME argument to
support options to named macros instead of hardcoding only LT_INIT
options.
* libltdl/m4/ltdl.m4 (_LTDL_CONVENIENCE): Factor common code from
LTDL_CONVENIENCE and LTDL_INIT([convenience]).
(_LTDL_INSTALLABLE): Factor out common code from LTDL_INSTALLABLE
and LTDL_INIT([installable]).
(_LTDL_SETUP): Renamed from LTDL_INIT.  Support new configure-time
options: --with-included-ltdl, --with-ltdl-lib,
--with-ltdl-include.
(LTDL_CONVENIENCE, LTDL_INSTALLABLE): Adjust.
(LT_WITH_LTDL): Removed.
(LTDL_INIT): Parse caller options.
* libltdl/m4/libtool.m4 (LT_INIT): Declare that this macro must be
expanded before LTDL_INIT, and be sure to parse caller options.
* configure.ac: Call directly into internal _LTDL_SETUP macro.
* libtoolize.m4sh (func_scan_files): Ltdl mode argument moved from
LT_CONFIG_LTDL_DIR to LT_INIT.
* tests/nonrecursive.at, tests/recursive.at: Use new interfaces.
* tests/configure-iface.at: Test it.
* Makefile.am (TESTSUITE_AT): Add configure-iface.at.
* doc/libtool.texi (Distributing libltdl): Document improved.
LTDL_INIT interfaces.
* NEWS: Updated.