If you use -MMD -MF then the correct .o filename is written to the
.*.d file as the compiler driver arranges everything.  This was done
in 19010:275abe1c5d24 for the hypervisor.

In this patch we do the same elsewhere in the xen-unstable tree,
particularly tools/.  Specifically:
 * Change tools/Rules.mk to add -MMD -MF ... to CFLAGS and set DEPS.
 * Remove -Wp,-MD... from every other Makefile
 * Remove setting of DEPS from every other Makefile
 * Ensure that every Makefile says -include $(DEPS)
 * Ensure that every Makefile's clean target removes $(DEPS)

Some Makefiles were already halfway there, but often for a different
variable name eg PROG_DEP.  The variable name is now standardised in
Rules.mk as DEPS.

I have done a test build with this change, on Debian etch.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>