opencv: please update to version 4.0

[gallafent]

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!

[luzpaz]

4.0.1 is latest stable

[reneeotten (Renee Otten)]

the openv4 port was added in ​this commit and is at its latest upstream version (currently 4.2.0)