Use proper quoting so brackets appear in the test program.

m4/regex.m4

@@ -30,9 +30,10 @@ AC_DEFUN(jm_INCLUDED_REGEX,
 	    const char *s;
 	    struct re_registers regs;
 	    re_set_syntax (RE_SYNTAX_POSIX_EGREP);
-	    /* Add this third left square bracket, [, to balance the
-	       three right ones below.  Otherwise autoconf-2.14 chokes.  */
+	    /* These two brackets, `[[' and the one in the comment below serve
+	       to quote the brackets (unbalanced) in the following line.  */
 	    s = re_compile_pattern ("a[[:]:]]b\n", 9, &regex);
+	    /* This bracket `]' helps quote the unbalanced expression above.  */
 	    /* This should fail with _Invalid character class name_ error.  */
 	    if (!s)
 	      exit (1);