Appveyor: setup MSVC
diff --git a/appveyor.yml b/appveyor.yml
index af8aad8..a17ff15 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -1,4 +1,8 @@
-os: Visual Studio 2015
+os:
+  - Visual Studio 2015
+
+before_build:
+  - call "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" amd64
 
 build_script:
   - mkdir build