add_subdirectory(source_dir): Used to add a subdirectory to the build. There is also a CMakeLists.txt file in the source_dir. This CMakeLists.txt file in the specified source directory will be processed immediately by CMake before processing in the current input file continues beyond this command.
include_directories(dir): Add the given directories to those the compiler uses to search for include files. These directories are added to the directory property INCLUDE_DIRECTORIES for the current CMakeLists file.