This patch fixes make install for Cygwin. Specifically,
it reverts to the previous behavior:

o install libpython$(VERSION)$(SO) in $(BINDIR)
o install $(LDLIBRARY) in $(LIBPL)

It also begins to remove Cygwin's dependency on
$(DLLLIBRARY) which I hope to take advantage of
when I attempt to make Cygwin as similar as possible
to the other Unix platforms (in other patches).

I tested this patch under Red Hat Linux 7.1 without
any ill effects.

BTW, I'm not the happiest using the following
test for Cygwin:

test "$(SO)" = .dll

I'm willing to update the patch to use:

case "$(MACHDEP)" in cygwin*

instead, but IMO that will look uglier.