Fix first-time make config=coverage test

Not sure why xargs -r is not the default...

Makefile

@@ -30,7 +30,7 @@ all: $(EXECUTABLE)
 
 ifeq ($(config),coverage)
 test: $(EXECUTABLE)
-	@find $(BUILD) -name '*.gcda' | xargs rm
+	@find $(BUILD) -name '*.gcda' | xargs -r rm
 	./$(EXECUTABLE)
 	@gcov -b -c $(BUILD)/src/pugixml.cpp.gcda | sed -e '/./{H;$!d;}' -e 'x;/pugixml.cpp/!d;'
 	@ls *.gcov | grep -v pugixml.cpp.gcov | xargs rm
@@ -51,4 +51,4 @@ $(BUILD)/%.o: %
 
 -include $(OBJECTS:.o=.d)
 
-.PHONY: all test clean	
+.PHONY: all test clean