Auto merge of #177 - servo:clang-cl, r=jdm

Use clang-cl to build.

This will be a hard requirement in forthcoming spidermonkey upgrades, so let's try and switch it on ahead of time.

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Author: bors-servo

appveyor.yml

@@ -32,6 +32,9 @@ environment:
 
   VCVARSALL_PATH: C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build
 
+  CC: "clang-cl.exe"
+  CXX: "clang-cl.exe"
+
   matrix:
     - TARGET: i686-pc-windows-msvc
       CROSS_COMPILE: 1