Some CMakeLists.txt cleanup

[dchengTSC]

This PR makes a few improvements to the CMakeLists.txt to make it work in our dev environment. We have mp4v2 as a submodule and use add_subdirectory(mp4v2) to make mp4v2 available to our CMake-based build system.

1. Use CMAKE_CURRENT_SOURCE_DIR instead of CMAKE_SOURCE_DIR, since our CMAKE_SOURCE_DIR isn't actually the mp4v2 folder. Added a similar change for CMAKE_BINARY_DIR.
2. Switched to target_include_directories, instead of using the global include_directories, which affects all targets. We have many targets in our CMake build system that this would also affect.
3. Adding PUBLIC and PRIVATE to the target_include_directories, which allows our CMakeLists to simply target_link_libraries(target PRIVATE mp4v2) and have CMake automatically add the correct include directories for mp4v2.

[enzo1982]

Looks good to me. Merged.

Thank you for this PR!