clang-tidy reporting unknown warnings

For me,

set(CMAKE_CXX_CLANG_TIDY "clang-tidy-8;--extra-arg=-Wno-error=unknown-warning-option")

solved the issue when running clang-tidy from within CMake.

Try appending -extra-arg=-Wno-unknown-warning-option to clang-tidy command line.

Using -extra-arg= asks clang-tidy to pass -Wno-unknown-warning-option to the underlying clang. Otherwise, it tries to interpret it as a clang-tidy flag.

Edit:

The run-clang-tidy.py script supports -extra-arg starting version 5.0. In prior versions, you'd need to edit the script and add that -extra-arg manually.

If you're on Ubuntu 16.04, you can get clang-tidy-5.0 from:
https://www.ubuntuupdates.org/package/xorg-edgers/xenial/main/base/clang-tidy-5.0