The internal smart javac wrapper is used by the new build system to build
the Java sources for the JDK using multiple cores and using a server.
It currently doubles the compile speed of the OpenJDK java sources.
More speedups are expected in the future.

It also tracks dependency information a recompiles dependent packages when
public apis have changed. It only regenerates header files if the native api
have changed.

It is not available in the final image. Thus to use it, you have to compile the OpenJDK yourself.
The new build system makes use of it, if you add --enable-sjavac to the configure arguments.