fnmatch, regex, fts: FALLTHROUGH consistency

fnmatch, regex, fts: FALLTHROUGH consistency

Be more consistent about how FALLTHROUGH is defined.
For Gnulib, use attribute.h.  For glibc, use __GNUC__ >= 7.
Problem for glibc reported by Vaseeharan Vinayagamoorthy in:
https://sourceware.org/pipermail/libc-alpha/2021-January/121778.html
* lib/fnmatch.c (FALLTHROUGH) [_LIBC]:
* lib/regex_internal.h (FALLTHROUGH) [_LIBC]:
Don’t worry about Clang, as it’s not needed and provokes GCC.
* lib/fts.c (FALLTHROUGH) [!_LIBC]:
* lib/regex_internal.h (FALLTHROUGH) [!_LIBC]:
Rely on attribute.h for FALLTHROUGH
* modules/regex: Depend on attribute module.