libtoolize: refactor macro_dir discovery around extract-trace script.

libtoolize: refactor macro_dir discovery around extract-trace script.

* libtoolize.m4sh (func_extract_trace): Function to call
$aux_dir/extract-trace.  The name and footprint are the same as
the core function in the extract-trace script file so that we
can choose to source that file when the option processing
becomes compatible with ours.
(require_ac_macro_dir, require_aclocal_amflags)
(require_am_macro_dir, require_macro_dir, require_makefile_am):
New functions factored out of func_scan_files for scanning and
setting macro_dir and friends.
(func_scan_files): Remove the factored out code to scan
configure.ac with sed, and then set macro_dir appropriately.
(func_serial_update_check, func_install_pkgmacro_subproject)
(func_install_pkgmacro_parent, func_install_pkgmacro_files)
(func_check_macros): Adjust.
* tests/libtoolize.at (Subproject ltdl without GNU M4): New test
to ensure `libtoolize --subproject' continues to work without a
GNU M4 dependency.
* NEWS: Updated.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>