Use CMAKE_CURRENT_SOURCE_DIR instead of CMAKE_SOURCE_DIR
This makes it possible to use OpenVolumeMesh inside another cmake project where CMAKE_SOURCE_DIR (the top level source directory of a project) is not the OpenVolumeMesh source directory.
This makes it possible to use OpenVolumeMesh inside another cmake project where CMAKE_SOURCE_DIR (the top level source directory of a project) is not the OpenVolumeMesh source directory.