During the bootstrapping of the toolchain and the final build header files from the IOKitUser and xnu projects are needed:

% cd ~/src/IOKitUser-179.7
% sudo make installhdrs DSTROOT=/tmp/IOKitUser.dst
% (cd /tmp/IOKitUser.dst; sudo pax -rw . /)

% cd ~/src/xnu-517.7.21
% sudo make installhdrs DSTROOT=/tmp/xnu_hdrs.dst
% (cd /tmp/xnu_hdrs.dst; sudo pax -rw . /)