[cmake] allow disabling Gui

[silverweed]

This Pull request:

Adds a gui build option that allows disabling the Gui library. It is meant to be used in conjunction with minimal=on as no effort is made to make sure that the option is coherent with the set of other options, meaning turning the gui off by itself won't automatically disable all its dependent modules. However, when minimal is on, everything works fine.

Rationale

When someone builds with minimal=on they don't care about graphics (otherwise they'd use gminimal) and building the graphics libraries is a waste of time. AFAIK, the gui already doesn't work when building with minimal=on, so we can spare time by not building it.

Ideally we should be able to also disable graf2d and graf3d, however too many core packages depend on them to be as easily doable (we'd have to disable half of the math/tree/hist modules and rethink modules.idx and hsimple.root), so that'll remain a wish for the future.

Checklist:

• tested changes locally
• updated the docs (if necessary)

[vepadulano]

Just a fly-by comment

the gui already doesn't work when building with minimal=on

I can confirm this

[github-actions]

Test Results

    21 files      21 suites   3d 16h 47m 31s ⏱️
 3 211 tests  3 209 ✅ 0 💤 2 ❌
65 699 runs  65 695 ✅ 2 💤 2 ❌

For more details on these failures, see this check.

Results for commit c61b5c9.