Improve clang-format check target by showing files which are not correctly formatted

[dariusarnold]

Hello, nice work creating/collecting all the examples.

I used your project to integrate clang-format into CMake. Doing that I adapted it slightly and want to suggest this as a possible improvement.
I found that the check target would just error if code was not formatted. In clang-format you can use the --dry--run and --Werror flags to get output of what was not formatted and also getting a sensible return code without grep.
My code for the custom target looks like this:

add_custom_target(clang-format-check
        COMMENT "Checking clang-format changes"
        COMMAND ${ClangFormat_EXECUTABLE}
        --style=file
        --dry-run
        --Werror
        ${ALL_SOURCE_FILES}
        )

The output when run: