Work around localcharset issue under OS X multi-threaded scenario.

* lib/localcharset.c.diff: New patch originally posted to Gnulib
<https://lists.gnu.org/archive/html/bug-gnulib/2012-12/msg00091.html>.
Fix handling of \x and \u escape sequences in Tcl.
Revert the last change for \x handling in Tcl.
Signal a change of the loaded catalogs in libintl_setlocale.
Allow user to supply custom sed command when generating en@quot.po.
Fix commit 417f0080823ae17d0f258ec08f3810cd0274e021.

AM_CONFIG_HEADER obsoletion (commit 0cde5e7f) has been mistakenly reverted
when merging the patch.
Update gnulib.
autogen.sh: Mention GNULIB_TOOL again for off-line bootstrap.
Fix string concatenation with '+' in Python.
Mention git submodule in Admin/release-steps.
Updated translations.
Updated translations.
Updated translations.
NEWS: Mention libasprintf change and Tcl bug fix.
Update from gnulib.
Port en@quot changes to gettext-tools.
tests/lang-javascript: Fix gjs module checks.
Fix typo in msgfmt-1 test.
Prepare for 0.18.3.
Regenerated for 0.18.3.