Sigh, build is broken again

Someone checked in code that assumes vsnprintf() exists.

Should be cool, we have an emulation right?  Well, no.
configure sets up to build port/snprintf.c if snprintf()
doesn't exist.  HPUX, for one, has that but not vsnprintf.

I guess the right fix is for configure to check for *both*
snprintf and vsnprintf --- anyone see a problem with that?
        regards, tom lane