appveyor.yml: Print versions of MSBuild and Ninja

This is useful to know what MSBuild.exe and Ninja we are using and
whether the correct one has been set via PATH by vcvarsall.bat and
hasn't been overwritten by appveyor.

.appveyor.yml

@@ -43,6 +43,7 @@ install:
 
 build_script:
   - cmd: echo No build step.
+  - cmd: if %backend%==ninja ( "C:\python34\ninja.exe" --version ) else ( MSBuild /version & echo. )
 
 test_script:
   - cmd: echo Running tests for %arch% and %compiler% with the %backend% backend