Fix #68: Add build of FSharp.Data.TypeProivders.dll to open source edition


This adds the build of FSharp.Data.TypeProviders on Linux/Mac, see
https://github.com/fsharp/fsharp/issues/68

The DLL might not yet be much use on Linux and Mac ut this at least gets the 'make' build on Linux/Mac par with the 'msbuild' build on Windows