On AIX 4.1, snprintf() is defined in libc.a but there's no prototype in

<stdio.h> or elsewhere.  Apparently it's not officially supported (though it
seems to work perfectly and IBM apparently uses it in internal code).  Anyhow,
if we omit our own snprintf() and vsnprintf() prototypes when we HAVE_SNPRINTF,
we get a billion warnings at compile time.  Use the C preprocessor to preprocess
stdio.h and make sure that there's actually a prototype.  Define
HAVE_SNPRINTF_PROTOTYPE if so, and use that to control our local prototype
definition.