add SYMBOL_VISIBILITY cache variable to match scons interface.

[enetheru]

Bring the 'cmake -LH' output inline with the scons output. I dont know what the policy on adding notes in comments are. I can ammend the commit to remove them if they are unwanted.

[dsnopek]

Ah, sorry, I didn't notice this when I approved: you need to squash this down to a single commit before I can merge it.

This is part of Godot's development workflow: https://docs.godotengine.org/en/latest/contributing/workflow/pr_workflow.html#modifying-a-pull-request

[enetheru]

Apologies, commits have been squashed and I'll remember it for next time.

[dsnopek]

Thanks!