-D_GNU_SOURCE kselftest breakage in mainline

Hi,

I'm seeing quite a lot of breakage in mainline as a result of daef47b89efd0b7 ("selftests: Compile kselftest headers with -D_GNU_SOURCE") and daef47b89efd0 ("selftests: Compile kselftest headers with -D_GNU_SOURCE") - thus far I've found that the use of static_assert() is triggering build breaks where testsuites aren't picking up the addition of _GNU_SOURCE (including stopping installing the other tests in the same directory), and there's a bunch of tests which #define _GNU_SOURCE in their code and now trigger build warnings. I'm looking at fixes and mitigations now.

The build failures are taking out the ALSA tests entirely which has caused my personal CI to explode badly :/

Thanks, Mark