qdbm : incompatible-java-bytecode-format with OpenJDK 7 as default-jdk

During a recent rebuild test it was detected that qdbm builds Java bytecode
which is only compatible with Java >= 7.

The default-jdk in Ubuntu Quantal has been changed to OpenJDK 7 and all
packages that Build-Depend on default-jdk are being tested to ensure that
they produce bytecode which is backwards compatible with earlier versions
of Java.

https://wiki.ubuntu.com/JavaTeam/Java7Default contains more details on how
to resolve this issue and make sure that the Java bytecode built by this
package is compatible with earlier Java runtime environments.

Note that lintian in Ubuntu Quantal also contains a experimental check
which can detect this problem (incompatible-java-bytecode-format). Enable
this by using the -E flag when running lintian.