Don't trip over outdated acinclude.m4.

Don't trip over outdated acinclude.m4.

* libtoolize.m4sh (all_pkgconfig_files, all_pkgmacro_files)
(all_pkgltdl_files): New variables listing all files ever
maintained by any release of libtoolize.
(func_install_pkgconfig_parent, func_install_pkgltdl_files)
(func_install_pkgconfig_subproject, func_install_pkgmacro_parent)
(func_install_pkgmacro_subproject): Carefully clean up any files
that may have been installed by previous libtoolize releases.
* tests/libtoolize.at: New test for libltdl/acinclude.m4 removal.
Reported by Andreas Schwab <schwab@suse.de>.