Windows install to custom location after building from source

[Tim Golden]

Martin v. Löwis wrote:
>> merge.py attempts to import config.py but I can't find it...
> 
> Just create an empty one.

Won't quite work: merge tries to find full_current_version
which is determined (if None) in msi.py from the rather
involved current version stuff.

I'm going to give up on this for tonight, but one possibility
is to turn msi.py into an importable module and for msilib
to import it and pull the config values from there.

TJG