Add a line at the start to tell GNU Emacs that this is a C++ source file.
drwxr-xr-x - ETL
drwxr-xr-x - autobuild
drwxr-xr-x - gtkmm-osx
drwxr-xr-x - synfig-core
drwxr-xr-x - synfig-docs
drwxr-xr-x - synfig-osx
drwxr-xr-x - synfig-studio