dev-java/gcj-jdk: Respect eselect-ecj, drop 4.7, bump 4.9, other fixes

dev-java/gcj-jdk: Respect eselect-ecj, drop 4.7, bump 4.9, other fixes

 * The ecj-gcj SLOT was previously hardcoded but it still used the
   version eselected at build time. Now it will resolve at runtime.
 * 4.7 is no longer needed as we have dropped ia64 support.
 * 4.9 has been bumped from 4.9.2. to 4.9.3.
 * Paths have been fixed for prefixed systems.
 * X flag has changed to awt to better reflect what it does.

Package-Manager: portage-2.2.20.1