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.

1 files changed, 2 insertions, 2 deletions

diff --git a/lib/fts.c b/lib/fts.c
index 8a9b5ed961..e6603f40e7 100644
--- a/lib/fts.c
+++ b/lib/fts.c
@@ -200,8 +200,8 @@ enum Fts_stat
     while (false)
 #endif
 
-#ifndef FALLTHROUGH
-# if (__GNUC__ >= 7) || (__clang_major__ >= 10)
+#ifdef _LIBC
+# if __GNUC__ >= 7
 #  define FALLTHROUGH __attribute__ ((__fallthrough__))
 # else
 #  define FALLTHROUGH ((void) 0)