cmake: Allow finding the CEF wrapper without a build folder on Linux

[tytan652]

Description

This modify the CEF finder of CMake 3.0 framework, to restore a behavior that was supported with 2.0.

The Flatpak (and also my CEF AUR packages), do not generate/use a build folder to provide the wrapper.

Motivation and Context

I don't see a real reason to drop this behavior.

How Has This Been Tested?

Makes Flatpak on CMake 3.0 with #9787 (with other fixes e.g. fix GIO finder, disabling AJA) build.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

• Tweak (non-breaking change to improve existing functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• If you have a shorter commit name idea, feel free to suggest it.
• I have included updates to all appropriate documentation.

[RytoEX]

The CMake 3.0 path isn't being used yet on Linux, is it? Just want to determine the urgency on this.

[PatTheMav]

The CMake 3.0 path isn't being used yet on Linux, is it? Just want to determine the urgency on this.

Correct - this is effectively a companion to the Linux update, but that PR's scope is limited to updating our own Ubuntu-based build process and has no changes specifically for Flatpak (because it doesn't update the manifest yet). This change is specifically required to make Flatpak work with those changes.