cmake: migrate variants '+gui' and '+docs' to subports

[mascguy (Christopher Nielsen)]

When enabling the variants for the CMake GUI and/or Docs, that results in a tremendous number of requisite dependencies built/installed first.

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.)

By making this change, we can also simplify the main port: It will eliminate most of the logic preventing circular dependencies, when the variants are enabled.

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