Add compatibility for non-GNU expr

Leading dashes in options confuses FreeBSD expr.

Author: jyn514

configure

@@ -66,7 +66,7 @@ while true; do
       ;;
 
     --readline=*)
-      FLAG_readline=$(expr "$1" : '--readline=\(.*\)')
+      FLAG_readline=$(expr \( "$1" : '--readline=\(.*\)' \) )
       ;;
     --readline)
       if test $# -eq 1; then