Attached is the patch I'd like to install, after more testing.

This patch affects the Windows build by removing src/doprnt.c.

This patch assumes that vsnprintf works.  This is true of the oldest,
cruftiest host I could get my hands on (a Solaris 8 box; Sun itself
stopped fixing Solaris 8 more than two years ago).  However, if Emacs
is still supposed to run on even-older (roughly, pre-1999) platforms
that lack vsnprintf then I can add the gnulib vsnprintf module, which
will provide a vsnprintf replacement for these ancient hosts.  I
assume that vsnprintf works on Windows, so the presence or absence of
the vsnprintf module shouldn't matter for Windows.