Silence gcc 7's fall-through warnings in `switch' blocks.

Instead of an intentionally missing `break' statement there must be a
comment line containing `fall through' (and nothing else) right before the
next `case' statement.

See

  https://stackoverflow.com/questions/45129741/gcc-7-wimplicit-fallthrough-warnings-and-portable-way-to-clear-them
6 files changed