fallthrough: update for GCC 7/8 * lib/quotearg.c (FALLTHROUGH): New macro. Use it whenever one switch case falls through into the next, replacing "/* Fall through */" comments. This exposed one instance of an unwarranted "fall through" comment: unwarranted because it preceded a "goto" label not a case statement. * lib/freopen-safer.c (freopen_safer): Likewise. * lib/fts.c (leaf_optimization_applies): Likewise. * lib/unistr/u8-uctomb-aux.c (u8_uctomb_aux): Likewise. * tests/test-getopt_long.h (getopt_long_loop): Likewise. * tests/test-tsearch.c (mangle_tree): Likewise. Also include tests/macros.h for the definition. * tests/test-argp.c (group1_parser): Likewise. * tests/test-getopt.h (getopt_loop): Likewise.