Use CMAKE_CURRENT_*_DIR instead of CMAKE_*_DIR.

This makes the library friendly to use as a git submodule within another
project, and should change nothing when compiled alone.

For example when having a directory structure like "my_project/external/libvnc",
where in libvnc contains a checkout of libvncserver, one can just reference that
directory from the CMakeLists.txt in my_project with

> add_directory ( external/libvnc )

and add `vncclient` / `vncserver` in my_project's `taret_link_libraries`, one can just
hack away without having to manually make / install LibVNCServer whenever
something is changed there.