We can't use ${WORLDTMP}/usr/{include,lib} for the compat layer. This
is because we populate these directories later, and a subsequent
-DNOCLEAN build may fail. So, we put them in
${WORLDTMP}/build/usr/{include,lib} instead and adjust Makefile.boot.

Again, this works on -stable and -current, but might break older
versions.

Submitted by: ru@