gettext: Restore patches to po/Makefile.in.in

gettext: Restore patches to po/Makefile.in.in

These were inadvertently lost during the conversion to Gnulib (gnulib:
Upgrade Gnulib and switch to bootstrap tool; commit 35b909062). The
files in po/gettext-patches/ can be imported using "git am" on top of
the gettext tag corresponding to AM_GNU_GETTEXT_VERSION in configure.ac
(currently 0.18.3). They handle translation of messages in shell files,
make msgfmt output in little-endian format, and arrange to use @SHELL@
rather than /bin/sh.

There were some changes solely for the purpose of distributing extra
files; for ease of maintenance, I've added these to
conf/Makefile.extra-dist instead.

Fixes: https://savannah.gnu.org/bugs/?57298
Signed-off-by: Colin Watson <cjwatson@ubuntu.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>