Set csound6 textdomain when reasonable

I would like the textdomain to be set when building csound, so that internationalization works out of the box. The attached patch works for me on linux, but I think this will require a few iterations to accomodate all use cases, which is why I submitted this as pull request so we can iterate on it instead of just merging it.

I believe windows would need a relative path because it is not known before install time, and don't know about mac.

So far the patch only changes anything when doing a release build on linux. On other builds, it could be set in the Custom.cmake