Problems with non Oracle boot JDKs and class file version 52

[David Holmes]

I've now seen two people encounter problems trying to build Profiles 
using the new build system, when their boot JDK is the locally installed 
IcedTea openjdk distribution (7 in one case and 6 in another).

The error occurs creating jar files eg

Creating resources.jar
Error occurred during initialization of VM
java/lang/UnsupportedClassVersionError: sun/misc/PostVMInitHook :
Unsupported major.minor version 52.0

Are we using bootclasspath tricks that only work on Oracle JDK?

David