MFC r219368:

r219368 (pjd):

To be able to use printf extensions we need to turn off gcc format checking.
Following the convention of NO_WERROR and NO_WCAST_ALIGN add NO_WFORMAT,
which, when defined in Makefile, turns off compile-time format checking
(by adding -Wno-format), but still allows to use high WARNS level.

Approved by: kib (co-mentor), pjd (mentor)