This makes it possible to define header files on the command line that
might include ${MACHINE} somewhere in the path. This might be used in
evbppc, for example, when defining PPC_PCI_MACHDEP_IMPL as, for example:
PPC_PCI_MACHDEP_IMPL="<arch/evbppc/sandpoint/pci_machdep.h>"
which will be included as
#include PPC_PCI_MACHDEP_IMPL
Prior to this change, the compile would fail trying to include
<arch/evbppc/1/pci_machdep.h>
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