CMAKE_SOURCE_DIR corresponds to the top project's source directory.
CMAKE_BINARY_DIR corresponds to the top project's binary directory.
The usage of these variables doesn't cause any problems when Opus is built as a standalone project.
This is not the case when Opus is added as submodule: the variables are set by the project that calls "add_subdirectory()".
The fix consists in using PROJECT_SOURCE_DIR and PROJECT_BINARY_DIR, which always refer to the current project.
Signed-off-by: Mark Harris <mark.hsj@gmail.com>
