This means that <ufs/*/*.h> come from ../../sys/ufs, but other
<sys/*> header files still come from the host system's /usr/include.
This slightly ugly hack to the Makefile means that makefs compiles on
-current and 1.5.x machines (and other systems), without requiring
major code rewrites in sys/ufs/ffs to allow sharing and without
requiring duplication of sys/ufs/ffs* under usr.sbin/makefs.
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