I wrote a concrete patch.

ISC License is discouraged by GNU, so this is 2-cluase BSDL.
http://www.gnu.org/licenses/license-list.html#ISC

diff --git a/BSDL b/BSDL
new file mode 100644
index 0000000..d63ffac
--- /dev/null
+++ b/BSDL
@@ -0,0 +1,24 @@
+
+  Copyright (C) 1993-2010 Yukihiro Matsumoto.
+  All rights reserved.
+
+  Redistribution and use in source and binary forms, with or without
+  modification, are permitted provided that the following conditions
+  are met:
+  1. Redistributions of source code must retain the above copyright
+     notice, this list of conditions and the following disclaimer.
+  2. Redistributions in binary form must reproduce the above copyright
+     notice, this list of conditions and the following disclaimer in the
+     documentation and/or other materials provided with the distribution.
+
+  THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+  IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+  WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+  DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR
+  ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+  DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+  OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+  HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+  STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
+  IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+  POSSIBILITY OF SUCH DAMAGE.
diff --git a/COPYING b/COPYING
index 9043404..a1f19ff 100644
--- a/COPYING
+++ b/COPYING
@@ -1,6 +1,6 @@
 Ruby is copyrighted free software by Yukihiro Matsumoto <matz / netlab.jp>.
-You can redistribute it and/or modify it under either the terms of the GPL
-version 2 (see the file GPL), or the conditions below:
+You can redistribute it and/or modify it under either the terms of the
+2-clause BSDL (see the file BSDL), or the conditions below:

   1. You may make and give away verbatim copies of the source form of the
      software without restriction, provided that you duplicate all of the
diff --git a/COPYING.ja b/COPYING.ja
index aa2a163..5b5b91c 100644
--- a/COPYING.ja
+++ b/COPYING.ja
<skipped; see [ruby-dev:42250]>

-- 
NARUSE, Yui  <naruse / airemix.jp>