Removed support for the gcc-2.[0-4] keywords dead and pure. They
stopped being used in /usr/src almost 2 years ago.

Don't pretend to support gcc-[3-infinity].

Added printf0like(). Feature tests for the printf0__ feature
are problematic, so this can't be used for the err() family yet

• it's only in recent versions of FreeBSD's hacked version of gcc.

Added comments about unused and *like().