Fix build for mingw32 on windows ($@ in macro)

Make using mingw32 on windows does not preserve $@ in macros
when they are modified using this pattern:
target: macro += something

This behaviour results in an error when QEMU_CFLAGS containing
"-MMD -MP -MT $@" is modified for compilation of source files
which use SDL: $@ will expand to nothing, -MT no longer has
the correct argument (it will take the next one from the command
line) and the build will fail or run with a wrong command line.

The problem is fixed by using a new macro QEMU_DGFLAGS
which is not modified by a target rule.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

rules.mak

@@ -11,16 +11,17 @@ MAKEFLAGS += -rR
 %.m:
 %.mak:
 
-QEMU_CFLAGS += -MMD -MP -MT $@
+# Flags for dependency generation
+QEMU_DGFLAGS += -MMD -MP -MT $@
 
 %.o: %.c $(GENERATED_HEADERS)
-	$(call quiet-command,$(CC) $(QEMU_CFLAGS) $(CFLAGS) -c -o $@ $<,"  CC    $(TARGET_DIR)$@")
+	$(call quiet-command,$(CC) $(QEMU_CFLAGS) $(QEMU_DGFLAGS) $(CFLAGS) -c -o $@ $<,"  CC    $(TARGET_DIR)$@")
 
 %.o: %.S
-	$(call quiet-command,$(CC) $(QEMU_CFLAGS) $(CFLAGS) -c -o $@ $<,"  AS    $(TARGET_DIR)$@")
+	$(call quiet-command,$(CC) $(QEMU_CFLAGS) $(QEMU_DGFLAGS) $(CFLAGS) -c -o $@ $<,"  AS    $(TARGET_DIR)$@")
 
 %.o: %.m
-	$(call quiet-command,$(CC) $(QEMU_CFLAGS) $(CFLAGS) -c -o $@ $<,"  OBJC  $(TARGET_DIR)$@")
+	$(call quiet-command,$(CC) $(QEMU_CFLAGS) $(QEMU_DGFLAGS) $(CFLAGS) -c -o $@ $<,"  OBJC  $(TARGET_DIR)$@")
 
 LINK = $(call quiet-command,$(CC) $(QEMU_CFLAGS) $(CFLAGS) $(LDFLAGS) -o $@ $(1) $(ARLIBS_BEGIN) $(ARLIBS) $(ARLIBS_END) $(LIBS),"  LINK  $(TARGET_DIR)$@")