Backport gettext fixes to get rid of warnings on macOS

Backport gettext fixes to get rid of warnings on macOS

Two fixes were committed recently to the gettext repo in order to make
gdb build warning-free on macOS.  This patch backports them both:

  - Make the format_arg attribute effective also in the case _INTL_REDIRECT_INLINE.
    113893dce80358a4ae0d9463ce73c5670c81cf0c
    http://git.savannah.gnu.org/cgit/gettext.git/commit/?id=113893dce80358a4ae0d9463ce73c5670c81cf0c

  - Enable the format_arg attribute also on clang on Mac OS X.
    bd6a52241c7c83c90e043ace2082a2508d273f55
    http://git.savannah.gnu.org/cgit/gettext.git/commit/?id=bd6a52241c7c83c90e043ace2082a2508d273f55

intl/ChangeLog:

	* libgnuintl.h (_INTL_MAY_RETURN_STRING_ARG, gettext, dgettext,
	dcgettext, ngettext, dngettext, dcngettext): Backport changes
	from upstream gettext.