fix warnings (#648)

* fix unknown command line argument

* fix warnings settings for gcc

* fix -Wzero-as-null-pointer-constant

* revert CMakeLists.txt

src/window/detail/GLFWWindow.cpp

@@ -99,7 +99,7 @@ static bool initialize_gl(int width, int height) {
 
 #ifndef __APPLE__
     glEnable(GL_DEBUG_OUTPUT);
-    glDebugMessageCallback(gl_message_callback, 0);
+    glDebugMessageCallback(gl_message_callback, nullptr);
 #endif
 
     glViewport(0, 0, width, height);
@@ -402,7 +402,7 @@ public:
             return;
         }
         cursor = shape;
-        // NULL cursor is valid for GLFW
+        // nullptr cursor is valid for GLFW
         glfwSetCursor(window, standard_cursors[static_cast<int>(shape)]);
     }