Support --release option to build JRuby extension for older platforms

[tagomoris]

Fixes #200

The patch can solve the problem msgpack/msgpack-ruby#239

Without this option (see Method java/nio/ByteBuffer.clear:()Ljava/nio/ByteBuffer; at 39):

$ javap -p -c -s -v -l -constants tmp/java/msgpack/org/msgpack/jruby/Encoder.class
(snip)
  private org.jruby.runtime.builtin.IRubyObject readRubyString();
    descriptor: ()Lorg/jruby/runtime/builtin/IRubyObject;
    flags: (0x0002) ACC_PRIVATE
    Code:
      stack=8, locals=2, args_size=1
         0: aload_0
         1: getfield      #15                 // Field runtime:Lorg/jruby/Ruby;
         4: new           #89                 // class org/jruby/util/ByteList
         7: dup
         8: aload_0
         9: getfield      #25                 // Field buffer:Ljava/nio/ByteBuffer;
        12: invokevirtual #78                 // Method java/nio/ByteBuffer.array:()[B
        15: iconst_0
        16: aload_0
        17: getfield      #25                 // Field buffer:Ljava/nio/ByteBuffer;
        20: invokevirtual #82                 // Method java/nio/ByteBuffer.position:()I
        23: aload_0
        24: getfield      #41                 // Field binaryEncoding:Lorg/jcodings/Encoding;
        27: iconst_0
        28: invokespecial #91                 // Method org/jruby/util/ByteList."<init>":([BIILorg/jcodings/Encoding;Z)V
        31: invokevirtual #94                 // Method org/jruby/Ruby.newString:(Lorg/jruby/util/ByteList;)Lorg/jruby/RubyString;
        34: astore_1
        35: aload_0
        36: getfield      #25                 // Field buffer:Ljava/nio/ByteBuffer;
        39: invokevirtual #98                 // Method java/nio/ByteBuffer.clear:()Ljava/nio/ByteBuffer;
        42: pop
        43: aload_1
        44: areturn
      LineNumberTable:
        line 69: 0
        line 70: 35
        line 71: 43
(snip)

With this option (see Method java/nio/ByteBuffer.clear:()Ljava/nio/Buffer; at 39 as well)

  private org.jruby.runtime.builtin.IRubyObject readRubyString();
    descriptor: ()Lorg/jruby/runtime/builtin/IRubyObject;
    flags: (0x0002) ACC_PRIVATE
    Code:
      stack=8, locals=2, args_size=1
         0: aload_0
         1: getfield      #15                 // Field runtime:Lorg/jruby/Ruby;
         4: new           #89                 // class org/jruby/util/ByteList
         7: dup
         8: aload_0
         9: getfield      #25                 // Field buffer:Ljava/nio/ByteBuffer;
        12: invokevirtual #78                 // Method java/nio/ByteBuffer.array:()[B
        15: iconst_0
        16: aload_0
        17: getfield      #25                 // Field buffer:Ljava/nio/ByteBuffer;
        20: invokevirtual #82                 // Method java/nio/ByteBuffer.position:()I
        23: aload_0
        24: getfield      #41                 // Field binaryEncoding:Lorg/jcodings/Encoding;
        27: iconst_0
        28: invokespecial #91                 // Method org/jruby/util/ByteList."<init>":([BIILorg/jcodings/Encoding;Z)V
        31: invokevirtual #94                 // Method org/jruby/Ruby.newString:(Lorg/jruby/util/ByteList;)Lorg/jruby/RubyString;
        34: astore_1
        35: aload_0
        36: getfield      #25                 // Field buffer:Ljava/nio/ByteBuffer;
        39: invokevirtual #98                 // Method java/nio/ByteBuffer.clear:()Ljava/nio/Buffer;
        42: pop
        43: aload_1
        44: areturn
      LineNumberTable:
        line 69: 0
        line 70: 35
        line 71: 43

[tagomoris]

I'll report on this pull request once the pre-released gem worked well.

[tagomoris]

We've confirmed that this change works well!
msgpack/msgpack-ruby#239 (comment)

[kou]

OK.
I'll merge this and release a new version.