Fix Config.mk's cc-option for -Wno-* options.

Fix Config.mk's cc-option for -Wno-* options.

These disable-warning options are handled specially by GCC:
 (a) they are ignored unless the compiler emits a warning; and
 (b) even then they produce a warning rather than an error

To handle this, modify the test invocation of GCC to compile a
fragment of code that will always provoke a warning (integer assigned
to pointer). This works around (a) above.

Then, we grep the compiler's stdout/stderr for the option-under-test,
the presence of which would indicate an "unrecognized command-line
option" warning/error. This works around (b) above, letting us
distinguish between the "integer assigned to pointer" and
"unrecognized command-line option" warnings.

Signed-off-by: Keir Fraser <keir@xen.org>