[MacPorts] #50815: dolfin @1.6.0_0: add vtk6 variant

[MacPorts]

#50815: dolfin @1.6.0_0: add vtk6 variant
----------------------------+--------------------------------
 Reporter:  mschamschula@…  |      Owner:  macports-tickets@…
     Type:  enhancement     |     Status:  new
 Priority:  Normal          |  Milestone:
Component:  ports           |    Version:  2.3.4
 Keywords:  haspatch        |       Port:  dolfin
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 Apparently dolfin fails to build under El Capitan since vtk5 currently
 cannot be installed. However, vtk6 a.k.a. vtk does properly install.

 See http://www.mail-archive.com/macports-
 users at lists.macosforge.org/msg39781.html

 I added a vtk6 variant (only test built it under Yosemite, as I don't have
 access to my El Cap build machine from home).

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Ticket URL: <https://trac.macports.org/ticket/50815>
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