* Qt: add options to disable docs and gui
- Add `~gui` option for minimal build
- Add `+doc` option to install docs, and attempt to disable the implicit
llvm dependency if not
- Removes the 'freetype' option which hasn't worked reliably in qt5, as
many of the gui components implicitly rely on freetype.
- Add and test version 5.15 (and skip qtlocation if disabling opengl)
- Refactor some of the dependency logic
I've tested this on linux with 5.15.2 and 4.8.7 in a couple of different
configurations.
* Address reviewer feedback and correctly disable llvm
* Fix qt doc generation