Added support for compiling BULLET with scons on windows

using bandoler's patch as a basis (thanks!).

A couple of notes:

-  This is for windows only, I did not have a chance to try linux yet.

-  SConscript for PHY_Bullet may need tweaking (plus support
 for other platforms), but at least it's in there :)

Any problems, shout :)

extern/SConscript

@@ -5,7 +5,8 @@ Import('user_options_dict')
 
 print "externs..."
 SConscript(['qhull/SConscript',
-            'solid/SConscript'])
+            'solid/SConscript',
+	    'bullet/SConscript'])
             
             
 if user_options_dict['USE_INTERNATIONAL'] == 1: