Merge gnulib getopt implementation into Emacs.

Merge gnulib getopt implementation into Emacs.

* Makefile.in (AUTOCONF_INPUTS): New macro.
($(srcdir)/configure, $(srcdir)/src/stamp-h.in): Depend on it,
so that these files also depend on m4/getopt.m4.
* configure.in: Configure getopt by including m4/getopt.m4,
and configuring a getopt replacement if necessary.
* make-dist: Add m4 subdirectory.  Unlink lib-src/getopt.h.
* m4/getopt.m4: New file.

* lib-src/Makefile.in (mostlyclean): Remove getopt.h, getopt.h-t.
(GETOPT_H): New macro, from gnulib.
(getopt.h): New rule, from gnulib.
(GETOPTOBJS): Now autoconfigured.
(GETOPTDEPS): getopt.h is now autoconfigured.
(getopt.o, getopt1.o): Depend on $(GETOPT_H), not ${srcdir}/getopt.h.
(getopt.o): Depend on ${srcdir}/gettext.h.
(movemail.o): Depend on $(GETOPT_H).
* lib-src/getopt.c, lib-src/getopt1.c: Sync from gnulib.
* lib-src/getopt_.h, lib-src/getopt_int.h, lib-src/gettext.h:
New files, from gnulib.
* lib-src/getopt.h: Removed (now is getopt_.h).

* nt/inc/gettext.h: Remove; no longer needed now that
lib-src/gettext.h exists.

* src/s/cygwin.h (C_SWITCH_SYSTEM): Remove, since gettext.h is
now part of lib-src.