netbsd-8 native epoxy and x11-links

[Greg Troxel <gdt%lexort.com@localhost>]

Recently, xentools411 stopped building on netbsd-8 because of problems
with native epoxy (in 2019Q2, x11-links did not include this; epoxy was
included on July 10).

  http://mail-index.netbsd.org/tech-x11/2019/12/18/msg002041.html

Manuel committed a workaround (to patch out trying to use epoxy), so
this issue is not pressing.

Perhaps netbsd-8 really is broken and will be fixed.  Assuming there is
a real issue, then clearly there will be NetBSD 8 systems out there that
don't have the fix, and pkgsrc must work on them.

So, it seems best to modify x11-links to not link in netbsd-8 native
epoxy.  While doing that, it seems good to avoid <=8 rather than ==8,
because it's confusing to have it withdrawn and reappear, and 7 is going
to be obsoleted soon anyway.

Using epoxy from base seems like a good change, on the assumption that
it is ok, which should be the case (but perhaps isn't).

Not understanding epoxy and the overall situation in depth, I am
somewhat hesitant, and we are in freeze.  So I propose without
conviction:

  do nothing now and hence for 2019Q4, because xentools411 has a
  workaround and we have no other reports of problems

  after the branch, modify pkgtools/x11-links/xorg.mk to omit epoxy on
  netbsd for versions <=8

  (Hope that the epoxy issues in 8 are resolved, and if so, consider
  having a way to detect that and including it again, while being
  mindful of the potential ssues of building on one kind of 8 while
  running on the other.)

This is definitely "please tell me if I am confused", rather than "I am
sure", and comments from people that understand more than me are hereby
solicited.