Improve "ant eclipse" to select right JRE for building

Whenever I run "ant eclipse" the setting choosing the right JVM is lost and has to be reassigned in the project properties.

In fact the classpath generator writes a new classpath file (as it should), but this onl ycontains the "default" entry:

<classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER"/>

Instead it should preserve something like:

<classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/jdk1.8.0_25"/>

We can either path this by a Ant property via command line or user can do this with "lucene/build.properties" or per user. An alternative would be to generate the name "jdk1.8.0_25" by guessing from ANT's "java.home". If this name does not exist in eclipse it would produce an error and user would need to add the correct JDK.

I currently have the problem that my Eclipse uses Java 7 by default and whenever I rebuild the eclipse project, the change to Java 8 in trunk is gone.

When this is fixed, I could easily/automatically have the "right" JDK used by eclipse for trunk (Java 8) and branch_5x (Java 7).