[MacPorts] #57010: freeciv @2.6.0: add sdl2 front end

[MacPorts]

#57010: freeciv @2.6.0: add sdl2 front end
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 Reporter:  kencu        |      Owner:  (none)
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:
 Keywords:               |       Port:  freeciv
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 freeciv can use `sdl2`, so we should make a variant to default to this on
 `10.6+`. We currently use `sdl1` and `gtk2` front ends, and it would be
 nice to keep the `sdl1` front end for the older `PPC` systems if we can.

 `sdl2` will likely need a new port, `sdl2_gfx`, to be added to MacPorts.
 See <http://www.ferzkopp.net/wordpress/2016/01/02/sdl_gfx-sdl2_gfx/>.

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Ticket URL: <https://trac.macports.org/ticket/57010>
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