Move the custom javac option to the Java subsystem.
Review Request #4141 — Created Aug. 6, 2016 and submitted
It was initially an option on ZincCompile. But putting it on Java allows us to have plugins depend on the custom javac instead of always depending on the JDK's tools.jar. This solves mismatch issues where we depend on the regular tools.jar when compiling a plugin, but then that plugin runs against the custom javac.
CI passes: https://travis-ci.org/pantsbuild/pants/builds/150446915