Use -Werror for all kernel configure tests

As a matter of fact, we're already using `-Werror` for most tests because of a bug in kernel-bio-empty-barrier.m4 which sets `-Werror` without reverting it afterwards. This meant that all tests which ran after this one was using `-Werror`.

This patch simply makes it clear that we're using `-Werror` and makes the code more readable and more predictable.