With print/pdftk which required GCJ (GNU Java) updated, the last
dependency on GCC 6 in the Ports Collection is gone, so we can remove
support for USE_GCC=6 and USE_GCC=6+. [1]

This does not remove lang/gcc6 yet, but helps avoid new dependencies,
and GCC 6 has been unmaintained upstream for more than a year.

On the way update two examples to use more current versions of GCC.

Thanks to: tobik [1]