manywarnings: update for GCC 7 * build-aux/gcc-warning.spec: * m4/manywarnings.m4 (gl_MANYWARN_ALL_GCC): Add GCC 7 warnings, notably -Wimplicit-fallthrough=5, which requires a non-comment fallthrough attribute. This is a bit cleaner than the comment versions. * lib/strftime.c, lib/dfa.c, lib/fnmatch.c, lib/mbrtowc.c: * lib/vasnprintf.c, tests/macros.h (FALLTHROUGH): New macro. Use it whenever one switch case falls through into the next.