vsvarsall in appveyor

appveyor.yml

@@ -39,6 +39,7 @@ install:
   - if [%COMPILER%]==[clang-cl] call C:\Miniconda36-x64\Scripts\activate.bat
   - if [%COMPILER%]==[clang-cl] conda config --add channels conda-forge --force
   - if [%COMPILER%]==[clang-cl] conda install --yes clangdev ninja cmake
+  - if [%COMPILER%]==[clang-cl] "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\vcvarsall.bat" x64
 
 before_build:
   - echo Running cmake...