mingw 64 build

The attached patch allows building a 64 bit Windows Postgres using the 
mingw64 compiler from 
<http://sourceforge.net/projects/mingw-w64/files/Toolchains%20targetting%20Win64/Automated%20Builds/>. 
It works both as a native compiler and for cross-compiling (which I 
tested on 32 bit Windows, but could in theory be done on any of the 
supported hosts, including Linux, Darwin and Cygwin).

The required changes are very modest, and I'd like to commit this so we 
can get some buildfarm coverage (I don't have an available 64 bit 
Windows machine for running a buildfarm member right now. but maybe 
someone else does.)

There will be some small consequent documentation changes.

cheers

andrew