Introduce DISTUTILS_USE_SDK as a flag to determine whether the

SDK environment should be used. Fixes #1508010.

msvccompiler.py

@@ -237,7 +237,7 @@ class MSVCCompiler (CCompiler) :
 
     def initialize(self):
         self.__paths = []
-        if os.environ.has_key("MSSdk") and self.find_exe("cl.exe"):
+        if os.environ.has_key("DISTUTILS_USE_SDK") and os.environ.has_key("MSSdk") and self.find_exe("cl.exe"):
             # Assume that the SDK set up everything alright; don't try to be
             # smarter
             self.cc = "cl.exe"