Examples: SDL+OpenGL2: Fixed Makefile. (#1886)

examples/example_sdl_opengl2/Makefile

@@ -26,7 +26,7 @@ ifeq ($(UNAME_S), Linux) #LINUX
 	ECHO_MESSAGE = "Linux"
 	LIBS = -lGL -ldl `sdl2-config --libs`
 
-	CXXFLAGS = -I../../  `sdl2-config --cflags`
+	CXXFLAGS = -I ../ -I../../  `sdl2-config --cflags`
 	CXXFLAGS += -Wall -Wformat
 	CFLAGS = $(CXXFLAGS)
 endif
@@ -35,7 +35,7 @@ ifeq ($(UNAME_S), Darwin) #APPLE
 	ECHO_MESSAGE = "Mac OS X"
 	LIBS = -framework OpenGL -framework Cocoa -framework IOKit -framework CoreVideo `sdl2-config --libs`
 
-	CXXFLAGS = -I../../ -I/usr/local/include `sdl2-config --cflags`
+	CXXFLAGS = -I ../ -I../../ -I/usr/local/include `sdl2-config --cflags`
 	CXXFLAGS += -Wall -Wformat
 	CFLAGS = $(CXXFLAGS)
 endif
@@ -44,7 +44,7 @@ ifeq ($(findstring MINGW,$(UNAME_S)),MINGW)
    ECHO_MESSAGE = "Windows"
    LIBS = -lgdi32 -lopengl32 -limm32 `pkg-config --static --libs sdl2`
 
-   CXXFLAGS = -I../../ `pkg-config --cflags sdl2`
+   CXXFLAGS = -I ../ -I../../ `pkg-config --cflags sdl2`
    CXXFLAGS += -Wall -Wformat
    CFLAGS = $(CXXFLAGS)
 endif