Small win32 build cleanup (versioninfo)

The previous build rules caused each binary to be re-linked on every
"make", even if nothing had changed. With this patch, it's only relinked
if it's actually updated.

//Magnus

PS. Yes, the old buildrule for the .rc file is still needed, as it's
used by pgevent.rc (or any other binary in the future that would need
it's own .rc file)

 <<win32_build_cleanup.patch>>