cmake: add options to disable gfx, test, example, ... targets (shader-slang#3572)

* cmake: add options to disable gfx, test and example targets

* cmake: enable gfx, test, example targets by default

* cmake: use same naming scheme

* cmake: option to disable slangd and slangc targets

* cmake: option for enabling slang-rt target

* cmake: option to deactivate llvm and glslang

* cmake: better option text

* cmake: remove duplicate slang-llvm option

* doc: update docs/building.md

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Co-authored-by: Yong He <yonghe@outlook.com>

Author: fknfilewalker