I just downloaded Visual Studio 2017 RC, which was released a few days ago and comes with support for C# 7. I can use C# 7 features from the IDE:
You can run "where csc.exe" from the visual studio 2017 command prompt and export the version of csc you need by adding it to the Path system variable. I was in a similar case where i needed to compile via command line and i ended up setting
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\MSBuild\15.0\Bin\Roslyn\
in system Path.