FALLTHROUGH warning in quotearg.c with gcc-7.1

[Assaf Gordon]

Hello,

Pulling the latest gnulib and compling with gcc-7.1,
I'm seeing the following warning:
====
gcc-7.1 -DLOCALEDIR=\"/usr/local/share/locale\" -DHAVE_CONFIG_H -I.  -Ilib -I./lib 
-Isrc -I./src   -g -O2 -MT lib/quotearg.o -MD -MP -MF $depbase.Tpo -c -o lib/quotearg.o 
lib/quotearg.c &&\
mv -f $depbase.Tpo $depbase.Po
lib/quotearg.c: In function 'quotearg_buffer_restyled':
lib/quotearg.c:61:23: warning: attribute 'fallthrough' not preceding a case 
label or default label
#  define FALLTHROUGH __attribute__ ((__fallthrough__))
                      ^
lib/quotearg.c:516:11: note: in expansion of macro 'FALLTHROUGH'
          FALLTHROUGH;
          ^~~~~~~~~~~
====

From a cursory check, it is not due to the recent commits:

 949b6cc fallthrough: reinstate a FALLTHROUGH instance in quotearg
 11fdf80 fallthrough: update for GCC 7/8
The warning appears even if I use an older revision of gnulib.

I haven't yet tried to investigate further,
but note this is with 'gnu datamash', not 'coreutils' - it's
possible there's a project-specific problem, not gnulib problem.

regards,
- assaf