Adrian Hall reported a problem to me that snprintf() doesn't exist in, at
least, Solaris 2.5.1.  We use it in backend/utils/adt/int8.c.

Add a check to configure so that we see if it exists or not, and, if not,
compile in snprintf.c from backend/port, which was taken from, and falls under
the same Berkeley license as us, the FreeBSD libc/stdio ...