regex: Fix "error: possibly undefined macro: gl_GLIBC21"

[Bruno Haible]

Now that module 'localcharset' no longer includes m4/glibc21.m4, I get
this error in coreutils:

autoreconf: running: aclocal -I m4 --force -I m4
configure.ac:63: warning: gl_GLIBC21 is m4_require'd but not m4_defun'd
m4/regex.m4:289: gl_PREREQ_REGEX is expanded from...
m4/gnulib-comp.m4:847: gl_INIT is expanded from...
configure.ac:63: the top level
autoreconf: configure.ac: tracing
configure.ac:63: warning: gl_GLIBC21 is m4_require'd but not m4_defun'd
m4/regex.m4:289: gl_PREREQ_REGEX is expanded from...
m4/gnulib-comp.m4:847: gl_INIT is expanded from...
configure.ac:63: the top level
autoreconf: configure.ac: not using Libtool
autoreconf: running: /darch/x86_64-linux-gnu/gnu/bin/autoconf --include=m4 
--force
configure.ac:63: warning: gl_GLIBC21 is m4_require'd but not m4_defun'd
m4/regex.m4:289: gl_PREREQ_REGEX is expanded from...
m4/gnulib-comp.m4:847: gl_INIT is expanded from...
configure.ac:63: the top level
configure:22484: error: possibly undefined macro: gl_GLIBC21

This fixes it:


2018-05-20  Bruno Haible  <address@hidden>

        regex: Fix "error: possibly undefined macro: gl_GLIBC21".
        * modules/regex (Files): Add m4/glibc21.m4.

diff --git a/modules/regex b/modules/regex
index 2d93c5e..1030f03 100644
--- a/modules/regex
+++ b/modules/regex
@@ -9,6 +9,7 @@ lib/regex_internal.h
 lib/regexec.c
 lib/regcomp.c
 m4/eealloc.m4
+m4/glibc21.m4
 m4/regex.m4
 m4/mbstate_t.m4