xgettext: Speed up the computation of the POT-Creation-Date.

xgettext: Speed up the computation of the POT-Creation-Date.

Reported by Serhii Tereshchenko, Arthur, Adam YS, Foucauld Degeorges
at <https://savannah.gnu.org/bugs/?66865>.

* gettext-tools/src/xgettext.c (has_some_mtimes, max_of_mtimes, some_mtimes_failed): Remove variables.
(files_for_vc_mtime): New variable.
(main): Initialize files_for_vc_mtime. Update files_for_vc_mtime instead of calling consider_vc_mtime.
(consider_vc_mtime): Remove function.
(read_exclusion_file, extract_from_file, extract_from_xml_file): Update files_for_vc_mtime instead of calling consider_vc_mtime.
(finalize_header): Call max_vc_mtime.
* NEWS: Mention the speedup.