[CMAKE] Fixed emcc warnings about SDL & OPENGL_LIBRARIES being nil

CMakeLists.txt

@@ -145,6 +145,8 @@ set(OXYGINE_LIBRARY_DIRS
 
 if (FORCE_GLES)
 	set(OPENGL_LIBRARIES libGLESv2.lib)
+else ()
+   set(OPENGL_LIBRARIES  "")
 endif(FORCE_GLES)
 
 
@@ -159,8 +161,10 @@ set(CORE_LIBS
 
 
 if (OX_USE_SDL2)
-	set(CORE_LIBS ${CORE_LIBS}
-		SDL2main SDL2)
+   if(!EMSCRIPTEN)
+	     set(CORE_LIBS ${CORE_LIBS}
+		      SDL2main SDL2)
+   endif(!EMSCRIPTEN)
 	set(OXYGINE_INCLUDE_DIRS ${OXYGINE_INCLUDE_DIRS} ${SDL2_INCLUDE_DIRS})
 endif(OX_USE_SDL2)