Ensure `make clean` is idempotent.

Prior to this, `make clean` would fail if the project was not already
fully built, and a second invokation would always fail.

examples/opengl2_example/Makefile

@@ -61,5 +61,5 @@ $(EXE): $(OBJS)
 	$(CXX) -o $(EXE) $(OBJS) $(CXXFLAGS) $(LIBS)
 
 clean:
-	rm $(EXE) $(OBJS)
+	rm -f $(EXE) $(OBJS)
 

examples/opengl3_example/Makefile

@@ -62,4 +62,4 @@ $(EXE): $(OBJS)
 	$(CXX) -o $(EXE) $(OBJS) $(CXXFLAGS) $(LIBS)
 
 clean:
-	rm $(EXE) $(OBJS)
+	rm -f $(EXE) $(OBJS)