reference System.Runtime in scripts to allow ExtensionAttribute to be…

… resolved when referencing PCL assemblies

fixes #381
closes #382

src/fsharp/build.fs

@@ -1575,9 +1575,12 @@ let DefaultBasicReferencesForOutOfProjectSources =
       // Note: this is not a partiuclarly good technique as it relying on the environment the compiler is executing in
       // to determine the default references. However, System.Core will only fail to load on machines with only .NET 2.0,
       // in which case the compiler will also be running as a .NET 2.0 process.
+      //
+      // NOTE: it seems this can now be removed now that .NET 4.x is minimally assumed when using this toolchain
       if (try System.Reflection.Assembly.Load "System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" |> ignore; true with _ -> false) then 
           yield "System.Core" 
 
+      yield "System.Runtime"
       yield "System.Web"
       yield "System.Web.Services"
       yield "System.Windows.Forms" ]
@@ -1604,6 +1607,7 @@ let SystemAssemblies primaryAssemblyName =
       yield "System.Web.Services"
       yield "System.Windows.Forms"
       yield "System.Core"
+      yield "System.Runtime"
       yield "System.Observable"
       yield "System.Numerics"]