win32 version info - try 2

Here's another attempt at adding version info to win32 stuff. Lots of
changes since last try :-)

* Build rules in Makefile.global builds the .o or .rc file required.
Each makefile defines PGFILEDESC with a file description that gets
imported.

* Full version number is pulled from pg_config.h. The numeric-only
version number is just in win32ver.rc - I notice the release
instructions have manual editing of the other rc file already, so it's
not that much more work. It can easily be changed by adding to
configure, but I'm not sure it's worth the job now that there is just
one place that would use it.

* Versioninfo not set on all DLLs, just the ones that would normally be
linked against client programs. I'd still like to see it on more, but
it's not *that* important. It could easily be added using the same rules
if we wanted to. It still uses the 7.5.x.y version number notation,
since for now it only affects pgevent.dll which is win32 only anyway and
doesn't have a version number of its own.

* Just two files to add - both go in src/port. (Moved the icon there
per discussion on IM with Bruce)

* A makefile can set PGAPPICON=<iconname> to specify an icon to be
linked in. (I don't know how icons work on other platforms, this could
perhaps be extended to others?)

//Magnus