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
Merge pull request #108 from funnelweb/issue68

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