Check for CEF_ROOT_DIR environment variable

[cpyarger]

Description

Adds a check for the CEF_ROOT_DIR environment variable, and if found uses it

Motivation and Context

Eases workload for local development

How Has This Been Tested?

Windows 10, Set Environment variable, CMake sees and uses it properly

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.
• I have included updates to all appropriate documentation.

[RytoEX]

This will likely conflict with #290, and I'm not sure we should continue using un-namespaced/un-prefixed environment variables.

[jp9000]

Going to close this for now because I'm not sure if we want to rely on environment variables anymore, plus we're reworking our CMake anyway. If there's a good argument for it I might consider it, although preferably after we merge the CMake overhaul.