CMake syntax fixes

Summary:

• add some more variables introduced in CMake 3.17
• allow back-slash characters in bracket comments
• detect _internal variables first, then built-ins

Reviewers: Framework: Syntax Highlighting, dhaumann

Reviewed By: Framework: Syntax Highlighting, dhaumann

Subscribers: dhaumann, kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D27929