Sources that are part of a unity build are normally not compiled
individually. However, the VS IDE allows a single source to be
compiled. This can also be achieved on the command line:
msbuild my.vcxproj ... -t:ClCompile -p:SelectedFiles=<src>
where `<src>` is the path in the vcxproj `ClCompile` entry.
In a target with precompiled headers, the source needs PCH settings to
support individual compilation even if the normal unity build does not.
