Make check-klayout an order-only prereq of merged GDS

The phony check-klayout target was a normal prerequisite of
$(GDS_MERGED_FILE), which forced the KLayout def2stream merge to re-run
on every `make finish`/`make metadata` invocation regardless of whether
the GDS inputs had changed. Moving it behind `|` keeps the KLayout
availability check when the GDS needs building, without marking the GDS
perpetually out-of-date.

Signed-off-by: Matt Liberty <mliberty@precisioninno.com>

Author: maliberty

flow/Makefile

@@ -673,7 +673,7 @@ $(WRAPPED_GDSOAS): $(OBJECTS_DIR)/klayout_wrap.lyt $(WRAPPED_LEFS)
 
 # Merge GDS using Klayout
 #-------------------------------------------------------------------------------
-$(GDS_MERGED_FILE): check-klayout $(RESULTS_DIR)/6_final.def $(OBJECTS_DIR)/klayout.lyt $(GDSOAS_FILES) $(WRAPPED_GDSOAS) $(SEAL_GDSOAS)
+$(GDS_MERGED_FILE): $(RESULTS_DIR)/6_final.def $(OBJECTS_DIR)/klayout.lyt $(GDSOAS_FILES) $(WRAPPED_GDSOAS) $(SEAL_GDSOAS) | check-klayout
 	$(UNSET_AND_MAKE) do-gds-merged
 
 .PHONY: do-gds-merged