Build: exclude -c flag from TF_CFLAGS

The -c flag should not be included in the global variable TF_CFLAGS;
it should be specified in the build rule only when its target is a
*.o file.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/make_helpers/build_macros.mk b/make_helpers/build_macros.mk
index 16bdd78..4ec060b 100644
--- a/make_helpers/build_macros.mk
+++ b/make_helpers/build_macros.mk
@@ -358,7 +358,7 @@
 else
 	@echo 'const char build_message[] = "Built : "$(BUILD_MESSAGE_TIMESTAMP); \
 	       const char version_string[] = "${VERSION_STRING}";' | \
-		$$(CC) $$(TF_CFLAGS) $$(CFLAGS) -xc - -o $(BUILD_DIR)/build_message.o
+		$$(CC) $$(TF_CFLAGS) $$(CFLAGS) -xc -c - -o $(BUILD_DIR)/build_message.o
 endif
 	$$(Q)$$(LD) -o $$@ $$(LDFLAGS) -Map=$(MAPFILE) --script $(LINKERFILE) \
 					$(BUILD_DIR)/build_message.o $(OBJS)