.i and .s files should be under build/ when KEEP_TEMPS=1

Makefile

@@ -301,10 +301,10 @@ ifneq ($(KEEP_TEMPS),1)
 	@echo "$(CC1) <flags> -o $@ $<"
 	@$(CPP) $(CPPFLAGS) $< | $(PREPROC) -i $< charmap.txt | $(CC1) $(CFLAGS) -o - - | cat - <(echo -e ".text\n\t.align\t2, 0") | $(AS) $(ASFLAGS) -o $@ -
 else
-	@$(CPP) $(CPPFLAGS) $< -o $*.i
-	@$(PREPROC) $*.i charmap.txt | $(CC1) $(CFLAGS) -o $*.s
-	@echo -e ".text\n\t.align\t2, 0\n" >> $*.s
-	$(AS) $(ASFLAGS) -o $@ $*.s
+	@$(CPP) $(CPPFLAGS) $< -o $(C_BUILDDIR)/$*.i
+	@$(PREPROC) $(C_BUILDDIR)/$*.i charmap.txt | $(CC1) $(CFLAGS) -o $(C_BUILDDIR)/$*.s
+	@echo -e ".text\n\t.align\t2, 0\n" >> $(C_BUILDDIR)/$*.s
+	$(AS) $(ASFLAGS) -o $@ $(C_BUILDDIR)/$*.s
 endif
 
 $(C_BUILDDIR)/%.d: $(C_SUBDIR)/%.c