- -Wstrict-prototypes is not available for C++, so don't try to
ignore it for C++.
- remove many _DIAGASSERT() checks against not NULL for functions
with arguments with nonnull attributes. in two cases, leave
code behind that should set defaults to "(null)".
- use -Wno-error=frame-address for i386 mcount, as it seems valid
to assume the caller will have a frame.fair