Actually, I was trying to reject non-warning options as argument to
-Werror=. However, the new test fails because -fdiagnostics-color=never is
always placed by the driver after the warning options when calling the compiler
proper. This patch prunes all -fdiagnostics-color from the command-line but the
last one, which is moved to the first position.

gcc/ChangeLog:

2015-09-24  Manuel López-Ibáñez  <manu@gcc.gnu.org>

	PR driver/67640
	* opts-common.c (prune_options): Discard all -fdiagnostics-color
	but the last one, which is moved to the front to be processed
	first.
	* opts.c (enable_warning_as_error): Reject options that do not
	control warnings.

gcc/testsuite/ChangeLog:

2015-09-24  Manuel López-Ibáñez  <manu@gcc.gnu.org>

	PR driver/67640
	* gcc.dg/Werror-13.c: New test.

From-SVN: r228094