fix Windows processor detection

Thanks to Martin Koller for this.

Modules/CMakeDetermineSystem.cmake

@@ -73,7 +73,7 @@ if(CMAKE_HOST_UNIX)
 else()
   if(CMAKE_HOST_WIN32)
     set (CMAKE_HOST_SYSTEM_NAME "Windows")
-    if (ENV{PROCESSOR_ARCHITEW6432})
+    if (DEFINED ENV{PROCESSOR_ARCHITEW6432})
       set (CMAKE_HOST_SYSTEM_PROCESSOR "$ENV{PROCESSOR_ARCHITEW6432}")
     else()
       set (CMAKE_HOST_SYSTEM_PROCESSOR "$ENV{PROCESSOR_ARCHITECTURE}")