We might e.g. be compiled in a container with old kernel headers. In this
scenario CLONE_PIDFD will work but pidfd_send_signal() might not be detected
because __NR_pidfd_send_signal is not defined because the kernel headers don't
match the kernel version.

This explains and fixes test-suite hangs on Jenkins I've recently debugged.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>