PATCH: Few changes regarding Windows build

Hi!

Please consider pushing these changes:

1. Windows Installer XML v.3.7 gets installed into a different
location than before, so an env.variable WIXDIR should be set up
properly.

2. I've got an error when building Installer target:
   C:\pgbuild\pgadmin3\pkg\win32\src\i18ndata.wxs(5): error LGHT0204:
   ICE21: Component: 'i18n_af_ZA' does not belong to any Feature.

Not sure, but probably it's ok to comment out that Directory tag,
because there are few others commented out (ar_SA, bg_BG, etc).

3. Mention MSVC++ Express Edition in the INSTALL document.

--
Best regards,
 J.F.