Update to the 20170627 snapshot of GCC 5.4.1.

Adjust JAVA_USES to require python (also at run time), not just
python:build.

And, most importantly, remove headers being created by GCC's
fixincludes machinery from the installation / packaging to avoid
breakage when FreeBSD's headers are changing afterwards. Several
fellow committers have strongly indicated that our headers do not
need the kind of fixing (or "fixing") that GCC performs.