pgsql: Link $(WIN32RES) into single-file modules only when PGFILEDESC i

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

Commit 0ffc201a51395ca71fe429ef86c872850a5850ee 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.

Branch
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/2fcedad3cac54ec01034f3e225dd7915e59ff8ff

Modified Files
--------------
src/makefiles/Makefile.win32 |    8 +++++++-
src/makefiles/pgxs.mk        |    2 +-
2 files changed, 8 insertions(+), 2 deletions(-)