getopt: merge _GL_UNUSED annotations from gnulib

getopt: merge _GL_UNUSED annotations from gnulib

gnulib now has annotations on at least some functions to cater to
compilation with -Wunused-parameter.  In order to follow suit cleanly,
I've added to libc-symbols.h some of the _GL_* macros that
gnulib-common.m4 puts into config.h.  (I don't think they belong in
sys/cdefs.h, at least not without further thought.)

At this point all gnulib-side changes to getopt.c have been merged.

	* include/libc-symbols.h: For gnulib compatibility, define
	_GL_UNUSED, _GL_UNUSED_LABEL, _GL_ATTRIBUTE_PURE, and
	_GL_ATTRIBUTE_CONST.
	* posix/getopt.c (_getopt_initialize): Mark argc and argv
	parameters with _GL_UNUSED.