configure: fix up word boundary regex logic now that we have gnulib

If we're using the bundled gnulib regex module, then assume word boundary
support is available to avoid issues with the regcomp test.  This also
unifies the different code paths a bit.

This fixes https://savannah.gnu.org/bugs/?50705.

configure.ac

@@ -604,19 +604,17 @@ AC_DEFINE_UNQUOTED(NANO_REG_EXTENDED, $nano_cv_flag_reg_extended,
 AC_MSG_CHECKING([for GNU-style word boundary regex support])
 AC_ARG_WITH(wordbounds,
 AS_HELP_STRING([--with-wordbounds], [Use GNU-style word boundary delimiters]),
-	[ case "$with_wordbounds" in
-	    no)
-		AC_MSG_RESULT(no)
-		;;
-	    *)
-		AC_MSG_RESULT(yes)
-		# We explicitly don't check if the user forced the option, because
-		# this is needed for cross compilers and we can't test the target.
-		AC_DEFINE(GNU_WORDBOUNDS, 1, [Define this if the system supports GNU-style word boundaries in regexes.]) gnu_wordbounds=yes,
-		;;
-	  esac
-	], [
-	    AC_TRY_RUN([
+	[with_wordbounds=$withval], [with_wordbounds=auto])
+if test "$with_wordbounds" != "no"; then
+    dnl If we're using the bundled gnulib regex module, we know it's supported.
+    if test "$ac_use_included_regex" = "yes"; then
+        with_wordbounds="yes"
+    fi
+
+    dnl We explicitly don't check if the user forced the option, because
+    dnl this is needed for cross compilers and we can't test the target.
+    if test "$with_wordbounds" != "yes"; then
+        AC_TRY_RUN([
 #ifdef HAVE_SYS_TYPES_H
 #include <sys/types.h>
 #endif
@@ -633,12 +631,23 @@ int main(void)
 	return 1;
     return 0;
 }],
-	    AC_MSG_RESULT(yes)
-	    AC_DEFINE(GNU_WORDBOUNDS, 1, [Define this if the system supports GNU-style word boundaries in regexes.]) gnu_wordbounds=yes,
-	    AC_MSG_RESULT(no),
-	    AC_MSG_WARN([*** Can't check for GNU-style word boundary support when cross-compiling])
-	)
-])
+        with_wordbounds="yes",
+        with_wordbounds="no",
+        with_wordbounds="cross")
+    fi
+fi
+case $with_wordbounds in
+yes)
+    AC_MSG_RESULT(yes)
+    AC_DEFINE(GNU_WORDBOUNDS, 1, [Define this if the system supports GNU-style word boundaries in regexes.])
+    ;;
+no)
+    AC_MSG_RESULT(no)
+    ;;
+cross)
+    AC_MSG_WARN([*** Can't check for GNU-style word boundary support when cross-compiling])
+    ;;
+esac
 
 if test x$color_support = xyes; then
 #    if test x$CURSES_LIB_NAME = xcurses; then