c: Introduce -Wfree-labels

c: Introduce -Wfree-labels

This is another recent GCC extension whose use is apparently
difficult to spot in code reviews.

The name of the option is due to Jonathan Wakely.  Part of it
could apply to C++ as well (for labels at the end of a compound
statement).

gcc/c-family/

	* c-opts.cc (c_common_post_options): Initialize
	warn_free_labels.
	* c.opt (Wfree-labels): New option.
	* c.opt.urls: Regenerate.

gcc/c/

	* c-parser.cc (c_parser_compound_statement_nostart): Use
	OPT_Wfree_labels for warning about labels on declarations.
	(c_parser_compound_statement_nostart): Use OPT_Wfree_labels
	for warning about labels at end of compound statements.

gcc/

	* doc/invoke.texi: Document -Wfree-labels.

gcc/testsuite/

	* gcc.dg/Wfree-labels-1.c: New test.
	* gcc.dg/Wfree-labels-2.c: New test.
	* gcc.dg/Wfree-labels-3.c: New test.