Merged changes from FSharp master to fix PCL78 interactive checker issues

This Fixes #162 PCL 78 based projects are not correctly resolved by the
Interactive checker

Changes come from here:
https://github.com/fsharp/fsharp/commit/8cbb262ceed1c74e75f138dfd354861e
b94858db

Ive tested with XS5.1 and my own build of the F# addin using this modified FCS and it works a treat :-)