Modernize gettext infrastructure

Modernize gettext infrastructure

Switch to use AM_GNU_GETTEXT, AM_GNU_GETTEXT_VERSION, and
AM_GNU_GETTEXT_REQUIRE_VERSION, this allows to stop bundling gettext
infrastructure files and let autoreconf invoke autopoint which will set
the gettext infrastructure up.

Use this opportunity to create separate .gitignore files for m4/ and po/
directories.

Tested with
autoreconf -if && ./configure --enable-maintainer-mode && make && make distcheck

Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>