In the folder "l2dc-core" there is a ".classpath" file that says :
"org.eclipse.jdt.launching.JRE_CONTAINER/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/JavaSE-1.6"
So let's suppose is JDK 6.
Also it seems that the original source link is dead, i may found one that i mention same revision but not mention "Optimal", i just uploaded it, so let me know:
https://zippyshare.day/tqlpkfGttYRShhC/file
And for those wondering how it would be the easiest way to build, i suggest a VM (virtualbox) with Ubuntu 14, since it includes openjdk 6.
https://releases.ubuntu.com/14.04/
https://releases.ubuntu.com/14.04/ubuntu-14.04.6-desktop-amd64.iso
And it would be easier to get a Eclipse version from that time:
Eclipse 2012:
https://www.eclipse.org/downloads/packages/release/indigo/sr2/eclipse-ide-java-developers