argmatch.h: new "_" definition may conflict with that of callers

[Jim Meyering]

Hi Akim,
I tried to build grep using latest gnulib and it failed with these errors:

  CC       grep.o
In file included from grep.c:30:
../lib/argmatch.h:35: error: "_" redefined [-Werror]
   35 | # define _(Msgid)  gettext (Msgid)
      |
In file included from grep.c:28:
system.h:43: note: this is the location of the previous definition
   43 | #define _(String) gettext(String)
      |
In file included from grep.c:30:
../lib/argmatch.h:36: error: "N_" redefined [-Werror]
   36 | # define N_(Msgid) (Msgid)
      |
In file included from grep.c:28:
system.h:42: note: this is the location of the previous definition
   42 | #define N_(String) gettext_noop(String)

First, the "N_" definition appears unnecessary: it is used in neither
of argmatch.[ch].
One fix for the "_" problem is to expand it: s/\b_\b/gettext /

Care to fix it, one way or another?
Thanks,
Jim