Add conditional to not build net46 in sourcebuild

[dseefeld]

A recent commit b16a488 to enable x-plat net46 build causes the source-build build to fail for 2.1.

[eerhardt]

/cc @nguerrera - who changed this recently: dseefeld@b16a488#diff-83d1b9d720377aaf70c666e9e75befd6

[nguerrera]

@weshaggard assured me this would work. The whole point of that commit was to get these binaries into source build.

[nguerrera]

Waiting to understand why we can't use the reference assembly package in source build as promised.

[nguerrera]

See dotnet/source-build#125 (comment)

We are using the same reference assembly packages as roslyn tools.

[eerhardt]

FYI - @weshaggard is OOF.

@nguerrera - can you help me understand where Microsoft.NETFramework.ReferenceAssemblies is hosted? I can't find it on nuget.org or on https://dotnet.myget.org/gallery/dotnet-core. Maybe to fix source-build we just need to add a new feed URL?

[crummel]

I think we'll also need to change MSBuild:
https://github.com/Microsoft/msbuild/blob/ac19036b0de4f1b58c3be76d51000df8db98560c/src/Directory.Build.props#L27
once we get the reference assemblies sorted out since SDK is referencing the MSBuild assemblies we produce.

[nguerrera]

AFAICT, we get Microsoft.NETFramework.ReferenceAssemblies from https://dotnet.myget.org/F/roslyn-tools/api/v3/index.json

[nguerrera]

It was my understanding from dotnet/source-build#125 that others in source-build were already using this.

[crummel]

I don't think anyone is now but I've got MSBuild building on its own with these changes. I'm going to try something similar with SDK now that that reference is unblocked if that looks reasonable.

[nguerrera]

@crummel, hmm that's a different package than the one from roslyn-tools. Wes specifically mentioned roslyn tools so I'm confused. But if we can get this building in source build with that, ok.

[nguerrera]

I think it would be good if source build could use Microsoft.NETFramework.ReferenceAssemblies because that one is based on the proposal that we intend to ship and it handles setting things up for you (no need to muck with FrameworkPathOverride, etc.)

[weshaggard]

Yes lets try to use Microsoft.NETFramework.ReferenceAssemblies and move it to the dotnet-core feed if we need to. I was pretty sure it was already being used as a pre-built but if it isn't then we can and should start doing so.