Change CMake behavior if CEF is not found

[tytan652]

Continuation of this obs-studio PR

Description

CMake will now emit a fatal error if CEF is not found rather than "disabling" obs-browser. Same if X11 is missing

Motivation and Context

Continuation of this obs-studio PR

Many Linux packagers doesn't seem to build OBS with obs-vst and obs-browser, or maybe they don't know about them. So I create this PR, to make the packager really decide to include or not those two.

How Has This Been Tested?

I replace the obs-studio's submodule with a symlink to my local repo without setting CEF_ROOT_DIR

Types of changes

• CMake changes

Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [x] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [ ] I have included updates to all appropriate documentation.