libtoolize: refactor aux_dir discovery.

libtoolize: refactor aux_dir discovery.

* libtoolize.m4sh (require_ac_aux_dir, require_aux_dir): New
functions factored out of func_scan_files for scanning and
setting aux_dir and friends.
(func_scan_files): Remove the factored out code to scan
configure.ac with sed, and then set macro_dir appropriately.
(func_install_pkgconfig_subproject, func_install_pkgconfig_parent)
(func_install_pkgmacro_files, func_check_macros): Adjust.

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