When the /Ze option is used (checked with _MSC_EXTENSIONS) all __STDC_* macros are not defined. I added __POCC_STDC_VERSION__ for internal use - which is always defined. It's currently undocumented, but I guess little will change about it in the future so I may as well document it.
I always assumed the few people using the extensions would know so anyway, but the /Zx option will get it's own symbol in RC5:
__POCC__EXTENSIONS=1