AppVeyor build fix

Author: wjakob

.appveyor.yml

@@ -11,7 +11,7 @@ install:
 build_script:
   - echo Running cmake...
   - cd c:\projects\nanogui
-  - cmake -G "Visual Studio 14 2015 Win64" -DPYTHON_EXECUTABLE:FILEPATH=C:/Python34-x64/python.exe
+  - cmake -G "Visual Studio 14 2015 Win64" -DPYTHON_EXECUTABLE:FILEPATH=C:/Python34-x64/python.exe .
   - set MSBuildLogger="C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
   - set MSBuildOptions=/v:m /p:Configuration=Release /logger:%MSBuildLogger%
   - msbuild %MSBuildOptions% nanogui.sln