Update SDL example for OS X

Just like GLFW example, use different settings in Makefile and
different shader versions.

demo/sdl/Makefile

@@ -15,7 +15,12 @@ ifeq ($(OS),Windows_NT)
 BIN := $(BIN).exe
 LIBS = -lmingw32 -lSDL2main -lSDL2 -lopengl32 -lm -lGLU32 -lGLEW32
 else
-LIBS = -lSDL2 -lGL -lm -lGLU -lGLEW
+	UNAME_S := $(shell uname -s)
+	ifeq ($(UNAME_S),Darwin)
+		LIBS = -lSDL2 -framework OpenGL -lm -lGLEW
+	else
+		LIBS = -lSDL2 -lGL -lm -lGLU -lGLEW
+	endif
 endif
 
 # Modes

demo/sdl/nuklear_sdl.c

@@ -27,12 +27,17 @@ static struct nk_sdl {
     struct nk_font_atlas atlas;
 } sdl;
 
+#ifdef __APPLE__
+  #define NK_SHADER_VERSION "#version 150\n"
+#else
+  #define NK_SHADER_VERSION "#version 300 es\n"
+#endif
 NK_API void
 nk_sdl_device_create(void)
 {
     GLint status;
     static const GLchar *vertex_shader =
-        "#version 300 es\n"
+        NK_SHADER_VERSION
         "uniform mat4 ProjMtx;\n"
         "in vec2 Position;\n"
         "in vec2 TexCoord;\n"
@@ -45,7 +50,7 @@ nk_sdl_device_create(void)
         "   gl_Position = ProjMtx * vec4(Position.xy, 0, 1);\n"
         "}\n";
     static const GLchar *fragment_shader =
-        "#version 300 es\n"
+        NK_SHADER_VERSION
         "precision mediump float;\n"
         "uniform sampler2D Texture;\n"
         "in vec2 Frag_UV;\n"