graphics/libGL 7.6.1 doesn't build with WITHOUT_NOUVEAU set

[Piotr Kubaj]

1. 8.0-RELEASE is unsupported, you should upgrade to 8.3-RELEASE or
9.0-RELEASE, since it may just not build on 8.0-RELEASE, as it's too
old.
2. WITHOUT_NOUVEAU knob is also unsupported. Read this
from /usr/ports/UPDATING:
20120421:
  AFFECTS: users of x11/xorg
  AUTHOR: x11 at FreeBSD.org
  
  The default mesa has been updated to 7.6.1 and the default xorg-server
  to 1.7.7.
  A switch is available for people that want to use mesa 7.11.x and
  xorg-server 1.10.6. This requires an nvidia card or Intel+KMS support.
  If you have an intel card and can try these experimental KMS support
  patches:
  10/head with a patch: http://people.freebsd.org/~kib/drm/
  FreeBSD stable/9 and releng/9.0:
  http://lists.freebsd.org/pipermail/freebsd-x11/2012-April/011640.html