CMake build should use target-level commands

Originally suggested in GitHub PR #386, the CMake build I wrote used include_directories, which has global side effects, instead of the more explicit target_include_directories, to include directories per target (and with private or public scoping).

Furthermore, it should also use CMAKE_CURRENT_SOURCE_DIR over CMAKE_SOURCE_DIR in order to allow inclusion in other projects via add_subdirectory(), and we can reduce the minimally required CMake version to 3.5 from 3.6.