Fixing the build on Linux(Ubuntu).

This was the work i put to make CppSharp compile on Ubuntu 14.04.1, mingw version 4.8.2 , mono version 3.10.0 , the purpose of the pull request is to help the maintainers, this pull request will show them how much work is needed to make it able to build on Linux. I have not verified that  build on other platforms is not broken after these changes except for Linux but it should not be.
decl.OrignalPtr will need a cast to int,  like new IntPtr((int)decl.OriginalPtr); because Inptr's constructor accepts int not pointer to int. But i went other root and too the refrence of decl.OriginalPtr because i think this is the intended purpose, as far as i have understand it.