Changes to the Windows build environment

[Dave Page <dpage(at)postgresql(dot)org>]

I've committed some changes to the Windows build environment to enable 
it to share dependencies with the PostgreSQL 8.3 build env. The 
following environment variables must now be set:

%WXWIN% - must point to the root of the wxWidgets installation.

%PGBUILD% - must point to the base directory containing libxml etc 
(previously, this was the pgadmin-deps directory)

%PGDIR% - must point to the root of the PostgreSQL installation.

Note that the pgInstaller version of PostgreSQL 8.3 is being built using 
VC++, thus the VC++ import library for libpq (libpq.lib) is installed 
into the PostgreSQL lib directory, not lib/ms. You may want to copy the 
library into the appropriate directory if building against a mingw built 
PostgreSQL.

Regards, Dave.