Even eclipse has dependency graph.Why it is building all the projects?

Scenario:

Project A, B and C is dependent on say X.jar. I am updating the X.jar where those files are only related to Project A. But eclipse builds all the three projects,if the jar gets updated.

Problem: Eclipse has all the dependencies graph(I believe) and why it is behaving like this?
If its build only project A then lot of time will be saved.

Worst case scenario will be when JDK changes (I am seeing this also as a JAR changes),so only the files which are compatiable with the latest versions and it dependencies shuld be built.

It should try for the incremental build . Any thoughts guys......