[MacPorts] #63830: cmake: migrate variants '+gui' and '+docs' to subports

[MacPorts]

#63830: cmake: migrate variants '+gui' and '+docs' to subports
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 Reporter:  mascguy  |      Owner:  mascguy
     Type:  defect   |     Status:  assigned
 Priority:  Normal   |  Milestone:
Component:  ports    |    Version:
 Keywords:           |       Port:  cmake
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 When enabling the variants for the CMake GUI and/or Docs, that results in
 a tremendous number of requisite dependencies built/installed first. And
 at some point, we may have a circular dependency issue, when those are
 enabled.

 Given that CMake is very much a foundational port, it should always have
 the bare minimum of dependencies. In order to ensure that, let's migrate
 the GUI and Docs to two separate subports. (Perhaps named `cmake-gui` and
 `cmake-docs`.)

 @michaelld, I'll do the work, if you support the idea!

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Ticket URL: <https://trac.macports.org/ticket/63830>
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