properly detect processor architecture on Windows

CMake is usually run as a 32 bit process even on 64 bit Windows, so the
PROCESSOR_ARCHITECTURE environment variable would always return x86. This post
gives a description on how to do it properly:

http://blogs.msdn.com/b/david.wang/archive/2006/03/26/howto-detect-process-bitness.aspx

Modules/CMakeDetermineSystem.cmake

@@ -73,7 +73,11 @@ if(CMAKE_HOST_UNIX)
 else()
   if(CMAKE_HOST_WIN32)
     set (CMAKE_HOST_SYSTEM_NAME "Windows")
-    set (CMAKE_HOST_SYSTEM_PROCESSOR "$ENV{PROCESSOR_ARCHITECTURE}")
+    if (ENV{PROCESSOR_ARCHITEW6432})
+      set (CMAKE_HOST_SYSTEM_PROCESSOR "$ENV{PROCESSOR_ARCHITEW6432}")
+    else()
+      set (CMAKE_HOST_SYSTEM_PROCESSOR "$ENV{PROCESSOR_ARCHITECTURE}")
+    endif()
   endif()
 endif()