libtoolize: refactor configure_ac discovery.

libtoolize: refactor configure_ac discovery.

Make a start on decomposing some of the giant functions at the
heart of libtoolize into the much more flexible and manageable
(albeit somewhat more verbose) model of self-organising
$require_<foo> function pointers.
* libtoolize.m4sh (require_configure_ac): New function factored
out of func_scan_files for setting $configure_ac appropriately,
implementation taken from extract-trace script.
(func_autoconf_configure): And similarly for ensuring that a
filename which matches something Autoconf would read also has
content that appears to be destined for Autoconf processing.
(func_scan_files): Simplified the factored out code to a single
require_configure_ac call.
(func_install_pkgconfig_files, func_check_macros): Ditto.
(seen_autoconf): Removed.

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