Add better error message when an in-source build is attempted. Add workaround for bootstrap build of wxWidgets due to cmake bug. Update minimum wxWidgets version to 3.0.0.

git-svn-id: https://svn.code.sf.net/p/freetel/code/codec2-dev@1345 01035d8c-6547-0410-b346-abe4f91aad63