Change build config to support the Core build system (as well)

[YoshiRulz]

This is a helpful first step towards #1415. It's also a requirement for GitLab CI, which uses Docker.

req:

• msbuild on Windows
• dotnet on Linux
• dotnet on Windows

Ready to squash, though trying the changes on more machines couldn't hurt.

bonus:

• msbuild on Linux (I think it will be deprecated soon anyway)
• double-check build tool replacement

[zeromus]

Please attach memorandum describing why gitlab CI matters

[jdpurcell]

I'd be against this if it has side effects in Visual Studio or MSBuild. After I moved everything into conditional imports it seems better but it needs more testing.

[jdpurcell]

It's behaving well with Visual Studio / MSBuild now. @YoshiRulz can you make sure it still works for you as of my most recent commit?

[YoshiRulz]

Does now.

edit for @zeromus: AppVeyor's OSS plan doesn't allow for concurrent jobs. When we get unit tests, GitLab would be able to run those on Debug while compiling Release.

edit 2: other than some warnings, dotnet from PS also works