Non-prefix safe ports

[Mikhail T.]

Building out a new machine from scratch using /opt (rather than the
usual /usr/local) as LOCALBASE, I found the following two ports, which
break:

  * multimedia/openh264 - the files built by the port get installed into
    stage/usr/local/... instead of stage/opt -- I had to manually move
    the files around before "make install" succeeded
  * graphics/gtk-update-icon-cache - at configure-stage, the port
    wrongly complaints, that the cairo installed on the system does not
    support X11. After I created a symlink: /usr/local -> ../opt, the
    problem went away.

I don't have the time right now to offer patches, so I'm not filing a PR
-- just raising awareness... Please, do the needful so the healing can
begin. Thank you. Yours,

    -mi