Examples: Apple: add Makefile for example_apple_metal, example_apple_opengl2. (#8637)

- Works only for macOS: use XCode for iOS
- It will produce a raw exe. It will not a produce an macOS application bundle (i.e a folder that includes exe and resources). To get app bundles, use the XCode project.
- An adjustment was required in main.mm (for macOS only), to ensure that the exe can run without any plist or storyboard under macOS

examples/example_apple_opengl2/Makefile

@@ -0,0 +1,25 @@
+IMGUI_DIR = ../../
+CXX = clang++
+CXXFLAGS = -std=c++17 -ObjC++ -Wall -Wextra -g -I$(IMGUI_DIR) -I$(IMGUI_DIR)/backends
+FRAMEWORKS = -framework Cocoa -framework OpenGL -framework GameController
+
+SOURCES = \
+    main.mm \
+    $(IMGUI_DIR)/backends/imgui_impl_osx.mm \
+    $(IMGUI_DIR)/backends/imgui_impl_opengl2.cpp \
+    $(IMGUI_DIR)/imgui.cpp \
+    $(IMGUI_DIR)/imgui_draw.cpp \
+    $(IMGUI_DIR)/imgui_demo.cpp \
+    $(IMGUI_DIR)/imgui_tables.cpp \
+    $(IMGUI_DIR)/imgui_widgets.cpp
+
+TARGET = imgui_example_apple_opengl2
+
+all: $(TARGET)
+
+$(TARGET): $(SOURCES)
+	$(CXX) $(CXXFLAGS) $(SOURCES) -o imgui_example_apple_opengl2 $(FRAMEWORKS)
+
+clean:
+	rm -f $(TARGET)
+	rm -rf $(TARGET).dSYM