Not all shells support [[ ]] compound commands; it's not in the most
recent Single UNIX Specification I could see, and the
ubuntu-clang-other-tests job is reporting
tools/validate-clang-check.sh: 18: [[: not found
Don't use [[ ]].
In addition, if you change extcap/etl.c, it tries to run clang-check on
it, but that file builds, and is only built, on Windows, so clang-check
fails dismally on UN*Xes. Omit it for now.