Makefile: use "-Werror" in cc-option

The "cc-option" macro should use "-Werror" to determine if a flag is
supported. Otherwise the test may not return a nonzero result. Also
conditionalize some of the warning flags which aren't supported by
clang.

Signed-off-by: Bill Wendling <morbo@google.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>