Fix for compilation issue when .NET 4.5 is installed

After I made this fix I could build Boo but 5 tests are failing for me on PC with .NET 4.5 and 1 on PC with .NET 4.0.

2 issues are related to GetMostSimilarTypeName.
2 issues are related to callable asynchronous handling (callables_49, callables_5) callables_49 cause issue for callables_5. callables_49 is also failing under pure .NET 4.0

Also BCE0149_external is failing.

So there is no changes for PC .NET 4.0 and it is much better for .NET 4.5.