btrfs.h was added to uapi in Linux 3.9. To fix building with older
header versions this adds a configure check for the header and re-adds
btrfs definitions to missing.h which was removed in
bed2e820 along with
two other ioctls used by gpt-auto-generator.
[ Apparently, btrfs.h was only added recently:
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=
55e301fd57a6239ec14b91a1cf2e70b3dd135194
let's re-add it for now -- kay ]