* 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.