build: remove more absolute paths from dependency tracking files

d6b12add90 ("DEPS handling: Remove absolute paths from references to
cwd") took care of massaging the dependencies of the output file, but
for our passing of -MP to the compiler to take effect the same needs to
be done on the "phony" rules that the compiler emits.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Ian Jackson <iwj@xenproject.org>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>

Config.mk

@@ -63,7 +63,7 @@ DEPS_INCLUDE = $(addsuffix .d2, $(basename $(wildcard $(DEPS))))
 DEPS_RM = $(DEPS) $(DEPS_INCLUDE)
 
 %.d2: %.d
-	sed "s! $$PWD/! !" $^ >$@.tmp && mv -f $@.tmp $@
+	sed "s!\(^\| \)$$PWD/! !" $^ >$@.tmp && mv -f $@.tmp $@
 
 include $(XEN_ROOT)/config/$(XEN_OS).mk
 include $(XEN_ROOT)/config/$(XEN_TARGET_ARCH).mk