uintptr_t for Datum

Attached patch is the part of the win64 patch that changes Datum to be
uintptr_t, and associated changes, with only very minor changes from
me. It also includes autoconf tests that I tricked Bruce into fixing
for me :-)

Comments?

Unless there are objections, I'll go ahead and apply this one for
broader buildfarm testing. It's working on all the platforms I could
test, and also on Bruces (where the original patch broke).

-- 
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/