Add --with-system-zlib in gas

Add --with-system-zlib in gas

This patch adds --with-system-zlib and remove --with-zlib in gas.

gas/

	* Makefile.am (ZLIBINC): New.
	(AM_CFLAGS): Add $(ZLIBINC).
	* as.c: (show_usage): Don't check HAVE_ZLIB_H.
	(parse_args): Likewise.
	* compress-debug.c: Don't check HAVE_ZLIB_H to include <zlib.h>.
	(compress_init): Don't check HAVE_ZLIB_H.
	(compress_data): Likewise.
	(compress_finish): Likewise.
	* configure.ac (AM_ZLIB): Removed.
	(zlibinc): New.  AC_SUBST.
	Add --with-system-zlib.
	* Makefile.in: Regenerated.
	* config.in: Likewise.
	* configure: Likewise.
	* doc/Makefile.in: Likewise.

gas/testsuite/

	* gas/i386/dw2-compress-1.d: Expect .zdebug_info.