Improved Ruby 2.0.* series patch which works without intervention with ruby-install.

readline.patch

--- a/ext/readline/extconf.rb
+++ b/ext/readline/extconf.rb
@@ -19,6 +19,10 @@ def readline.have_func(func)
   return super(func, headers)
 end

+def readline.have_type(type)
+  return super(type, headers)
+end
+
 dir_config('curses')
 dir_config('ncurses')
 dir_config('termcap')
@@ -93,4 +97,10 @@ readline.have_func("remove_history")
 readline.have_func("clear_history")
 readline.have_func("rl_redisplay")
 readline.have_func("rl_insert_text")
+unless readline.have_type("rl_hook_func_t")
+  # rl_hook_func_t is available since readline-4.2 (2001).
+  # Function is removed at readline-6.3 (2014).
+  # However, editline (NetBSD 6.1.3, 2014) doesn't have rl_hook_func_t.
+  $DEFS << "-Drl_hook_func_t=Function"
+end
 create_makefile("readline")
--- a/ext/readline/readline.c
+++ b/ext/readline/readline.c
@@ -1883,7 +1883,7 @@ Init_readline()

     rl_attempted_completion_function = readline_attempted_completion_function;
 #if defined(HAVE_RL_PRE_INPUT_HOOK)
-    rl_pre_input_hook = (Function *)readline_pre_input_hook;
+    rl_pre_input_hook = (rl_hook_func_t *)readline_pre_input_hook;
 #endif
 #ifdef HAVE_RL_CATCH_SIGNALS
     rl_catch_signals = 0;

This is based upon @riocampos's: https://gist.github.com/riocampos/b2669b26016207224f06 which in turn is a more generalised version forked from @mislav.