Allow unix platforms to build executables

[eltone]

Add a unix only msbuild config to locate mono when building the net47 target.

[jamescourtney]

Thanks for the contribution. How do I test this? Is this a pattern that's established elsewhere? It all looks quite reasonable, but this is firmly outside my area of expertise.

[eltone]

This was taken from the f# type provider project. I have seen it used in a lot of SDKs.

You should be able to test it in a linux VM or with WSL.

[jamescourtney]

Thanks! Closing now.