Use ninja on appveyor

[janisozaur]

ninja is not only faster than msbuild, but also brings some sanity to
the messages produced by compiler: first, it holds a lock over output,
only allowing single process to write at once; second: it reduces
verbosity to something palatable.

msbuild:

  GraphViewStyle.cpp
C:\projects\sourcetrail\src\lib\component\view\GraphViewStyle.cpp(148): warning C4244: '=': conversion from 'float' to 'int', possible loss of data [C:\projects\sourcetrail\build\win64\Sourcetrail_lib.vcxproj]
C:\projects\sourcetrail\src\lib\component\view\GraphViewStyle.cpp(319): warning C4244: '=': conversion from 'size_t' to 'float', possible loss of data [C:\projects\sourcetrail\build\win64\Sourcetrail_lib.vcxproj]
C:\projects\sourcetrail\src\lib\component\view\GraphViewStyle.cpp(319): warning C4244: '=': conversion from 'float' to 'int', possible loss of data [C:\projects\sourcetrail\build\win64\Sourcetrail_lib.vcxproj]
C:\projects\sourcetrail\src\lib\component\view\GraphViewStyle.cpp(335): warning C4244: '=': conversion from 'size_t' to 'float', possible loss of data [C:\projects\sourcetrail\build\win64\Sourcetrail_lib.vcxproj]
C:\projects\sourcetrail\src\lib\component\view\GraphViewStyle.cpp(335): warning C4244: '=': conversion from 'float' to 'int', possible loss of data [C:\projects\sourcetrail\build\win64\Sourcetrail_lib.vcxproj]
C:\projects\sourcetrail\src\lib\component\view\GraphViewStyle.cpp(360): warning C4244: '=': conversion from 'size_t' to 'float', possible loss of data [C:\projects\sourcetrail\build\win64\Sourcetrail_lib.vcxproj]
C:\projects\sourcetrail\src\lib\component\view\GraphViewStyle.cpp(360): warning C4244: '=': conversion from 'float' to 'int', possible loss of data [C:\projects\sourcetrail\build\win64\Sourcetrail_lib.vcxproj]
C:\projects\sourcetrail\src\lib\component\view\GraphViewStyle.cpp(621): warning C4244: '=': conversion from 'int' to 'float', possible loss of data [C:\projects\sourcetrail\build\win64\Sourcetrail_lib.vcxproj]
  MainView.cpp
C:\projects\sourcetrail\src\lib\component\view\GraphViewStyle.cpp(691): warning C4244: '=': conversion from 'int' to 'float', possible loss of data [C:\projects\sourcetrail\build\win64\Sourcetrail_lib.vcxproj]
C:\projects\sourcetrail\src\lib\component\view\GraphViewStyle.cpp(728): warning C4244: 'return': conversion from 'double' to 'int', possible loss of data [C:\projects\sourcetrail\build\win64\Sourcetrail_lib.vcxproj]
C:\projects\sourcetrail\src\lib\component\view\GraphViewStyle.cpp(733): warning C4244: 'return': conversion from 'double' to 'int', possible loss of data [C:\projects\sourcetrail\build\win64\Sourcetrail_lib.vcxproj]

ninja:

[244/398] Building CXX object CMakeFiles\Sourcetrail_lib.dir\src\lib\component\view\GraphViewStyle.cpp.obj
..\..\src\lib\component\view\GraphViewStyle.cpp(148): warning C4244: '=': conversion from 'float' to 'int', possible loss of data
..\..\src\lib\component\view\GraphViewStyle.cpp(319): warning C4244: '=': conversion from 'size_t' to 'float', possible loss of data
..\..\src\lib\component\view\GraphViewStyle.cpp(319): warning C4244: '=': conversion from 'float' to 'int', possible loss of data
..\..\src\lib\component\view\GraphViewStyle.cpp(335): warning C4244: '=': conversion from 'size_t' to 'float', possible loss of data
..\..\src\lib\component\view\GraphViewStyle.cpp(335): warning C4244: '=': conversion from 'float' to 'int', possible loss of data
..\..\src\lib\component\view\GraphViewStyle.cpp(360): warning C4244: '=': conversion from 'size_t' to 'float', possible loss of data
..\..\src\lib\component\view\GraphViewStyle.cpp(360): warning C4244: '=': conversion from 'float' to 'int', possible loss of data
..\..\src\lib\component\view\GraphViewStyle.cpp(621): warning C4244: '=': conversion from 'int' to 'float', possible loss of data
..\..\src\lib\component\view\GraphViewStyle.cpp(691): warning C4244: '=': conversion from 'int' to 'float', possible loss of data
..\..\src\lib\component\view\GraphViewStyle.cpp(728): warning C4244: 'return': conversion from 'double' to 'int', possible loss of data
..\..\src\lib\component\view\GraphViewStyle.cpp(733): warning C4244: 'return': conversion from 'double' to 'int', possible loss of data

[mlangkabel]

Looking at the build times of our CI for this pull request it doesn't seem to speed up building. But I would like to pull this change nevertheless, because it opens up support for another build system. And if people want to keep using msbuild, that's still possible.