printf0 is originally a FreeBSD extension. clang has adopted it for all
supported versions. The FreeBSD-specific gcc versions have it. The stock
gcc versions, including ports, do not. bcc and tcc follow stock gcc
behavior. For cases we know don't implement it, define it away so do
not get warnings with -Wsystem-headers. My testing for 67d1a1cd9e77
didn't test that case, so I introduced a regression.
All these compilers need to be considered because __printf0like is used
in err.h and stdlib.h. Since it's used in system headers, it has to
work on all the compilers we support on FreeBSD, not just the ones that
can build FreeBSD itself.
Noticed by: jhb
Fixes: 67d1a1cd9e77
Sponsored by: Netflix