x86/EFI: work around CFLAGS being passed in through environment

Short of a solution to the problem described in
http://lists.xen.org/archives/html/xen-devel/2012-12/msg00648.html,
deal with the bad effect this together with c/s 25751:02b4d5fedb7b has
on the EFI build by filtering out the problematic command line items.

Signed-off-by: Charles Arnold <carnold@xxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/efi/Makefile
+++ b/xen/arch/x86/efi/Makefile
@@ -5,7 +5,7 @@ obj-y += stub.o
 create = test -e $(1) || touch -t 199901010000 $(1)
 
 efi := $(filter y,$(x86_64)$(shell rm -f disabled))
-efi := $(if $(efi),$(shell $(CC) $(filter-out $(CFLAGS-y),$(CFLAGS)) -c check.c 2>disabled && echo y))
+efi := $(if $(efi),$(shell $(CC) $(filter-out $(CFLAGS-y) .%.d,$(CFLAGS)) -c check.c 2>disabled && echo y))
 efi := $(if $(efi),$(shell $(LD) -mi386pep --subsystem=10 -o check.efi check.o 2>disabled && echo y))
 efi := $(if $(efi),$(shell rm disabled)y,$(shell $(call create,boot.init.o); $(call create,runtime.o)))