Fix path updates to not store the lower() version of the original path values.
Various additional build fixes and tweaks.

- Validate py_prefix and py_include_dir
- Add install_dir/bin to the PATH
- Set CMAKE_PREFIX_PATH to install_dir
- If QTDIR is set then use it for ALTERNATIVE_QT_INCLUDE_DIR
Merge pull request #13 from RobinD42/more-build-tweaks

More build tweaks and fixes