possible cases for SOURCE_DIR and GIT_URL
SOURCE_DIR GIT_URL
abs yes ERROR
abs no OK, local project, no clone: SOURCE_DIR/CMakeLists.txt
rel yes OK, clone_dir/SOURCE_DIR/CMakeLists.txt
rel no OK, local project, no clone: parent_path(deps.cmake)/SOURCE_DIR/CMakeLists.txt
no yes OK, clone_dir/CMakeLists.txt
no no ERROR
Such projects could be specified without GIT_URL:
possible cases for SOURCE_DIR and GIT_URL SOURCE_DIR GIT_URL abs yes ERROR abs no OK, local project, no clone: SOURCE_DIR/CMakeLists.txt rel yes OK, clone_dir/SOURCE_DIR/CMakeLists.txt rel no OK, local project, no clone: parent_path(deps.cmake)/SOURCE_DIR/CMakeLists.txt no yes OK, clone_dir/CMakeLists.txt no no ERROR