build-sys: Append -Werror when testing flags

build-sys: Append -Werror when testing flags

Clang don't treat unknown warnings flags as an error, but rather as a
warning. The result is that the detection for whic CFLAGS are supported
by this compiler will not work, since the compilation will succeed.

With this patch we now successfully detect clang doesn't support
-Wlogical-op, as opposed to previous behavior:

	checking if clang supports flag -Wlogical-op in envvar CFLAGS... no

We use this macro only for LDFLAGS and CFLAGS, so it's safe to stash
-Werror there.