The configure script checks for linux/btrfs.h which is only available since
3.9 (55e301fd57a6239ec: Btrfs: move fs/btrfs/ioctl.h to
include/uapi/linux/btrfs.h).

It now also uses static_assert which is only available since GCC 4.6, so
handle it as well in the legacy patch.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>