Building Eclipse itself is horror, because it is not at all clear what needs to happen to actually develop and build it. This should, once, become a reasonable description of how to - for instance - add a fix to the compiler. But for now it's just a list of links and snippets.
Building a subdirectory from the Aggregator clone:
cd eclipse.jdt.core/org.eclipse.jdt.core mvn -Pbuild-individual-bundles clean verify