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

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.

Also, define strcasecmp() and strncasecmp() here all the time since we define
our own versions and always use them.  The only way we could get into trouble
doing this would be if the vendor's versions didn't have the same parameters,
but I don't see that happening.  If we don't define them here, we get warnings
all over the place that default prototypes are being used for 'em, and the
system header to include varies from OS to OS.