Use rm -rf to remove build dir, so it doesn't error out if dir doesn't exist.

stm/Makefile

@@ -193,6 +193,6 @@ $(BUILD)/compile.o: $(PYSRC)/grammar.h
 $(BUILD)/emitbc.o: $(PYSRC)/emit.h
 
 clean:
-	/bin/rm -r $(BUILD)
+	/bin/rm -rf $(BUILD)
 
 .PHONY: all clean

unix-cpy/Makefile

@@ -86,6 +86,6 @@ $(BUILD)/emitcpy.o: $(PYSRC)/emit.h
 $(BUILD)/emitbc.o: $(PYSRC)/emit.h
 
 clean:
-	/bin/rm -r $(BUILD)
+	/bin/rm -rf $(BUILD)
 
 .PHONY: clean

unix/Makefile

@@ -95,6 +95,6 @@ $(BUILD)/emitcpy.o: $(PYSRC)/emit.h
 $(BUILD)/emitbc.o: $(PYSRC)/emit.h
 
 clean:
-	/bin/rm -r $(BUILD)
+	/bin/rm -rf $(BUILD)
 
 .PHONY: clean