[MacPorts] #47218: protobuf3-cpp: update to 3.0.0-alpha-2

[MacPorts]

#47218: protobuf3-cpp: update to 3.0.0-alpha-2
--------------------------+---------------------------
 Reporter:  ryandesign@…  |      Owner:  blair@…
     Type:  update        |     Status:  new
 Priority:  Normal        |  Milestone:
Component:  ports         |    Version:  2.3.99
 Keywords:  haspatch      |       Port:  protobuf3-cpp
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 This patch updates the protobuf3-cpp port to version 3.0.0-alpha-2. It
 also fixes the `github.setup` line and removes a bunch of lines that then
 become unnecessary, and no longer overrides the version number so that the
 version the user sees as being installed is the correct one. (The port
 previous claimed to have installed version "0.0.0.1" when it actually
 installed version "3.0.0-alpha-1".)

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Ticket URL: <https://trac.macports.org/ticket/47218>
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