Changes to the Windows build environment

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.