Blender Git Loki

Clang-Tidy: error out when executable not found

Stop with an error when the Clang-Tidy executable cannot be found.

Without this check, CMake will happily report "Found Clang-Tidy" but with
the fallback version (0, 0, 0), when `CLANG_TIDY_EXECUTABLE` points to a
non-existing executable.