[Patch] Windows installer: allow manually setting WIXDIR as appropriate.

Hi all,

How about allowing wix3 to be located in a different path, pointed to by
setting WIXDIR environment variable _manually_ before building.

The following patch allows doing so, while fully retaining previous
behaviour in case this variable was not manually set.

(E.g. when building in a 32bit windows, the "Program Files (x86)" is not
good enough, and I'd rather not modify the scripts for just setting a
correct path)

--- pkg/win32/Make.bat.orig Fri Sep 25 13:20:24 2015
+++ pkg/win32/Make.bat Fri Oct 02 21:55:27 2015
@@ -9,7 +9,9 @@
SET APPKEYWORDS=PostgreSQL, pgAdmin
SET APPCOMMENTS=PostgreSQL Tools
SET APPDESCRIPTION=Management and administration tools for the
PostgreSQL DBMS
+IF NOT "%WIXDIR%"=="" GOTO DONE_WIXDIR
SET WIXDIR="C:\Program Files (x86)\Windows Installer XML v3\bin"
+:DONE_WIXDIR

SET BUILDTREE="../.."

Thank you,
Nikolai