Fix for when cocos is included as a cmake subproject.

Projects seeking to include cocos2d-x as a subproject can now define COCOS2D_ROOT to enable this behaviour. For example my master cmake file:

    cmake_minimum_required(VERSION 2.8 FATAL_ERROR)
    project(game)
    
    set(CMAKE_C_FLAGS_DEBUG   "-g -Wall -Werror -O0 -D_DEBUG -DCOCOS2D_DEBUG=1")
    set(CMAKE_CXX_FLAGS_DEBUG "-std=c++11 -g -Wall -Werror -O0 -D_DEBUG -DCOCOS2D_DEBUG=1")
    set(CMAKE_C_FLAGS_RELEASE   "-Wall -Werror -O3 -D_DEBUG -DCOCOS2D_DEBUG=0")
    set(CMAKE_CXX_FLAGS_RELEASE "-std=c++11 -Wall -Werror -O3 -D_DEBUG -DCOCOS2D_DEBUG=0")

    list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/lib/cocos2d-x/cmake/Modules")

    set(ENV{COCOS_CONSOLE_ROOT} "${CMAKE_SOURCE_DIR}/lib/cocos2d-x/tools/cocos2d-console/bin")
    set(ENV{COCOS_X_ROOT} "${CMAKE_SOURCE_DIR}/lib/cocos2d-x")
    set(COCOS2D_ROOT "${CMAKE_SOURCE_DIR}/lib/cocos2d-x")

    add_subdirectory(lib/cocos2d-x)
    add_subdirectory(src/game)