MFC r350739-r350740 (by cem)

r350739:
Disable useless -Wformat-zero-length

It is part of -Wformat, which is enabled by -Wall. Empty format strings are
well defined and it is perfectly reasonable to expect them in a formatting
interface.

r350740:
r350739 try #2

For some inexplicable reason, C++ compilers reject the -Wno- flag, and also
(ab)use CWARNFLAGS.

Reported by: imp