[MacPorts] #57729: opencv: please update to version 4.0

[MacPorts]

#57729: opencv: please update to version 4.0
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Reporter: gallafent | Owner: stromnov
Type: update | Status: assigned
Priority: Normal | Milestone:
Component: ports | Version: 2.5.4
Keywords: | Port: opencv
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As per [https://opencv.org/opencv-4-0-0.html], OpenCV version 4.0 was
release on 2018-11-20, and the 3.4 branch is now in maintenance mode.
Please update the port to build version 4.0 by default. I imagine there
may be people that require 3.4 rather than 4.0 for the time being, which
might lead to the port forking into opencv3 and opencv (or whatever the
convention here is) 
 I guess any such people should make themselves known
in this ticket, if retaining 3.4 is required.

Depending on speed of progress of this ticket, I may end up having a go at
adapting the existing portfile myself. If I do, I'll report progress here.

Thanks!