fixed #3517, make CC_USE_PHYSICS can be switch off, and bring Scene::update(float), Scene::addChild(...) out of CC_USE_PHYSICS segment
issue #3517, enable CC_USE_PHYSICS by default
add Scene::update & addChild into CC_USE_PHYSICS scope, TestCpp compiles and runs successfully when CC_USE_PHYSICS == 0
Merge pull request #4688 from walzer/develop

make CC_USE_PHYSICS can ACTUALLY be switch off