diff --git a/Makefile b/Makefile
index c75df1cf..e0cef2f1 100644
--- a/Makefile
+++ b/Makefile
@@ -327,12 +327,12 @@ endif
 	@$(check_echo) "    [SP]     $<"
 	$(Q)$(check) $(CFLAGS) $(CHECKER_FLAGS) $<
 	@echo "    [CC]     $@"
-	$(Q)$(CC) $(CFLAGS) -c $< -o $@ $($(subst -,_,$(@:%.o=%)-cflags)) \
+	$(Q)$(CC) $(CFLAGS) -c $< -o $@ $($(subst /,_,$(subst -,_,$(@:%.o=%)-cflags))) \
 		$($(subst -,_,btrfs-$(@:%/$(notdir $@)=%)-cflags))
 
 %.static.o: %.c
 	@echo "    [CC]     $@"
-	$(Q)$(CC) $(STATIC_CFLAGS) -c $< -o $@ $($(subst -,_,$(@:%.static.o=%)-cflags)) \
+	$(Q)$(CC) $(STATIC_CFLAGS) -c $< -o $@ $($(subst /,_,$(subst -,_,$(@:%.static.o=%)-cflags))) \
 		$($(subst -,_,btrfs-$(@:%/$(notdir $@)=%)-cflags))
 
 all: $(progs_build) $(libs_build) $(BUILDDIRS)