example: sdl2+opengl3: fixes for Makefile

- append CXXFLAGS instead of overwriting them
- add glad.c build rule
- also handle apple and mingw

examples/example_sdl_opengl3/Makefile

@@ -35,12 +35,11 @@ CXXFLAGS += -I../libs/gl3w
 
 ## Using OpenGL loader: glew
 ## (This assumes a system-wide installation)
-# CXXFLAGS = -lGLEW -DIMGUI_IMPL_OPENGL_LOADER_GLEW
+# CXXFLAGS += -lGLEW -DIMGUI_IMPL_OPENGL_LOADER_GLEW
 
 ## Using OpenGL loader: glad
-## (You'll also need to change the rule at line ~77 of this Makefile to compile/link glad.c/.o)
 # SOURCES += ../libs/glad/src/glad.c
-# CXXFLAGS = -I../libs/glad/include -DIMGUI_IMPL_OPENGL_LOADER_GLAD
+# CXXFLAGS += -I../libs/glad/include -DIMGUI_IMPL_OPENGL_LOADER_GLAD
 
 ##---------------------------------------------------------------------
 ## BUILD FLAGS PER PLATFORM
@@ -50,7 +49,7 @@ ifeq ($(UNAME_S), Linux) #LINUX
 	ECHO_MESSAGE = "Linux"
 	LIBS += -lGL -ldl `sdl2-config --libs`
 
-	CXXFLAGS += -I../libs/gl3w `sdl2-config --cflags`
+	CXXFLAGS += `sdl2-config --cflags`
 	CFLAGS = $(CXXFLAGS)
 endif
 
@@ -59,7 +58,7 @@ ifeq ($(UNAME_S), Darwin) #APPLE
 	LIBS += -framework OpenGL -framework Cocoa -framework IOKit -framework CoreVideo `sdl2-config --libs`
 	LIBS += -L/usr/local/lib -L/opt/local/lib
 
-	CXXFLAGS += -I../libs/gl3w `sdl2-config --cflags`
+	CXXFLAGS += `sdl2-config --cflags`
 	CXXFLAGS += -I/usr/local/include -I/opt/local/include
 	CFLAGS = $(CXXFLAGS)
 endif
@@ -68,7 +67,7 @@ ifeq ($(findstring MINGW,$(UNAME_S)),MINGW)
    ECHO_MESSAGE = "MinGW"
    LIBS += -lgdi32 -lopengl32 -limm32 `pkg-config --static --libs sdl2`
 
-   CXXFLAGS += -I../libs/gl3w `pkg-config --cflags sdl2`
+   CXXFLAGS += `pkg-config --cflags sdl2`
    CFLAGS = $(CXXFLAGS)
 endif
 
@@ -88,6 +87,9 @@ endif
 %.o:../libs/gl3w/GL/%.c
 	$(CC) $(CFLAGS) -c -o $@ $<
 
+%.o:../libs/glad/src/%.c
+	$(CC) $(CFLAGS) -c -o $@ $<
+
 all: $(EXE)
 	@echo Build complete for $(ECHO_MESSAGE)