For projects excluded from a solution configuration, still include the ActiveCfg

[KirillOsenkov]

Right now when a project is not included in a solution configuration, it's property set in the project configuration doesn't include either ActiveCfg or Build.0 - it's missing entirely.

Unfortunately this messes with Visual Studio 2017 for Windows :( VS Win insists on having ActiveCfg for all configurations, but to avoid building a project in a configuration it wants to omit the Build.0 entry.