unknown conversion %m

All or almost all the warnings seen on Windows/Mingw of the type 
"warning: unknown conversion type character 'm' in format" come from 
checking of three functions: errmsg, elog and errdetail. I therefore 
propose to disable the attribute checking of those three functions, on 
Windows only (since that's the only place I've seen the warnings). 
That's a much more conservative change than I made previously which 
would have turned off all format warnings on Mingw, and along with 
fixing the INT64_FORMAT (see email just sent) would fix the vast 
majority of compiler warnings, so we'd be almost clean again on MinGW.

cheers

andrew