hxcs command options

[skial]

These two small changes allow the user to control which csc compiler gets used by hxcs.

1. Using -D csharp-compiler=C:/path/to/csc now works on windows. Before, this resulted in hxcs looking for a .bat file.
2. Using -D NET_35 will stop findMsvc looking for a newer csc on a matched version, in this case .net framework 3.5.

I'm not entirely comfortable with the -D NET_ change, but its a lot simpler for the user to define than looking up the path to csc, especially if they have never done any C# before.

Are there any foreseeable problems that these changes might raise?