[MacPorts] #43785: merge zmq ports, remove zmq20

[MacPorts]

#43785: merge zmq ports, remove zmq20
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 Reporter:  michaelld@…  |      Owner:  merijn@…
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:  2.2.1
 Keywords:               |       Port:  zmq, zmq20, zmq22, zmq-devel
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 I propose that we merge the devel/zmq* ports into a single Portfile, and
 that we remove zmq20 since that is the ZeroMQ dev's recommendation (use
 2.2 minimum). I'm attaching a diff to the current svn trunk with my
 proposed changes. This new combined Portfile seems to work for me, and is
 a lot simpler to maintain. I haven't tried the +pgm variant yet, but it
 seems consistent with all prior Portfile's usage so it should work just as
 before.

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Ticket URL: <https://trac.macports.org/ticket/43785>
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