[RFA] Eliminate warnings about snprintf declaration

[DJ Delorie]

> Almost - you don't actually need them in libiberty/configure.ac,
> unless you see warnings about them during the build of libiberty.
> To make GDB -Werror happy, all you need is gdb/configure.ac and
> include/libiberty.h.

Libiberty doesn't currently build with -Werror, but it's probably a
good idea to ac_test for everything in libiberty.h anyway.  It serves
as both a sanity check and an example to others.