gcc can recognize various regexps in comments. Since C++17 provides [[fallthrough]], using /* fallthrough */ consistently seems appropriate until we switch to C++17. see also https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
Patch taken (and forward-ported to HEAD) from <https://anonscm.debian.org/cgit/pkg-multimedia/ardour.git/plain/debian/patches/0100-fix-typos.patch>