Link $(WIN32RES) into single-file modules only when PGFILEDESC is set.

Commit 0ffc201a included this object
unconditionally.  Being unprepared for that, most external, single-file
modules failed to build.  This better aligns the GNU make build system
with the heuristic in the MSVC build's Project::AddDirResourceFile().
In-tree, installed modules set PGFILEDESC, so they will see no change.
Also, under PGXS, omit the nonfunctioning rule to build win32ver.rc.
Back-patch to 9.5, where the aforementioned commit first appeared.