Filter unsupported Objective-C options, to avoid
'unknown-warning-option' warnings when using Clang:

  [34/373] Compiling Objective-C object libcommon.fa.p/audio_coreaudio.m.o
  warning: unknown warning option '-Wold-style-declaration'; did you mean '-Wout-of-line-declaration'? [-Wunknown-warning-option]
  warning: unknown warning option '-Wimplicit-fallthrough=2'; did you mean '-Wimplicit-fallthrough'? [-Wunknown-warning-option]
  2 warnings generated.

Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Tested-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>