some header files (e.g., <err.h>) include <machine/something.h>, and this
will not pick up the right header files, so it may be removed eventually
anyway. But some people who are not willing to build the right way
apparantly want this, so this is for them.
anything other than <sys/*.h>), and unnecessary in most cases. (The
situations where it is necesary can be dealt with by manually-made symlinks,
which is acceptable since they should only occur during testing. Remember:
the tree does not compile well if you do not have matching header files
installed. Half-baked -I directives don't cover enough of the cases.)
