build CSharp bindings in SOURCE_DIR

On my linux machines, building the csharp bindings did not complete because they were build in the BINARY_DIR, but later steps looked for them in SOURCE_DIR . It seems the standard for openbabel bindings is to build in the SOURCE_DIR , so I modified so they build there and the build now completes on my linux machines using mcs as the CSHARP_EXECUTABLE .

I don't have access to a windows machine, so someone should make sure this doesn't break windows, where I think it is much more important to have working csharp bindings.