build: java: Add openjdk 1.6 support

I noticed, because I tested on a machine somewhere deep down in some
basement, covered in dust, which was running Debian stable.

Debian stable at this time still uses 1.6 ... sigh ... and because of
that, I thought: "Why not include support for that?" So here you are.

Signed-off-by: Bart Kuivenhoven <bemkuivenhoven@gmail.com>