Enforce proper definition of __warn_references in sys/cdefs.h
Now, __warn_references defined only if it was not yet defined. Actually, it should be undefined (if it was already defined in Google's headers) and redefined properly. This change could break many things since it will issue many (currently silenced) warnings, and for those projects using -Werror it will break build. Thus, this change should be thoroughly tested even though it looks so simple.