Problematic dependency in plpython Makefile [Windows]

On Windows, src/pl/plpython/Makefile has a rule whose target line expands to
something like "python33.def: C:/Windows/system32/python33.dll".  When doing a
MinGW build with Cygwin's make-3.81, that line elicits an error:

Makefile:69: *** target pattern contains no `%'.  Stop.

Seeing a second colon, make treats the line as a static pattern rule.  Perhaps
the MinGW project ships a make patched to avoid this, or perhaps folks
building PostgreSQL override WINDIR.  In any event, that dependency is not
useful: we can't build the named file if it's absent, and an error from
pexports is a good as an error from make.  Let's drop the dependency.

Note that this affects --without-python builds during "make clean".

Thanks,
nm