Add a helper to externalize projects

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
javac is an external project now

As of daf72cd(Add a .gitignore file), javac lives in

	https://github.com/fiji/javac

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Ignore if javac is cloned into external/

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Adjust .gitignore

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>