I'm a bit leery of this idea, as it is painful to write and debug reliable
preprocessor macros. When you need macros you really need them, but they're best
avoided in the first place.
The fanciest macros I've contributed to Gnulib are in <intprops.h>; they had to
be macros because C won't let you write polymorphic functions. A quick look at
the P99 package suggests it wouldn't have helped me write or debug the
intprops.h macros, and that it's intended more for meta-macro programming,
something that gets pretty hairy pretty quickly and something I'm not sure I'd
like to encourage. (Reading the warning for P99_IS_EMPTY gives me the willies,
for example.)
That being said, if there's a need for a particular macro for something
Gnulibish then of course we should be all ears.
