gcc enables the warnings with -Wformat, but the association between
varargs function and format checking is done by the following (on FreeBSD):

int	 asprintf __P((char **, const char *, ...)) __printflike(2, 3);

where

#define	__printflike(fmtarg, firstvararg) \
	    __attribute__((__format__ (__printf__, fmtarg, firstvararg)))


it's the __attribute__ cluster that's built-in to gcc.
Linux uses different #defines but the result is similar.
hence the need for ifdefs somewhere to avoid feeding __stuff__ to other compilers.
(there are many many more __things__.)
the list of formats is also built-in to the compiler.
it doesn't look as though it can easily be extended, but i have waded
through neither code nor spent time down by the docs.

Plan 9's compilers use a pragma to allow sets of format types
and flags to be defined outside the compiler, in a way that doesn't
give other compilers heartburn,
and thus without absolutely requiring ifdefs.  to be fair, the current
implementation assumes all flags (in the current scope) have the same meaning,
and does define a handful of built-in flags such as '.' (although neither restriction
is essential); 375 lines.