Added osx check for copy method

Checking whether the platform is OSX. OSX has a ulimit (maximum allowed open file descriptors) of 1024. When asynchronous copying of files happens, this limit can be reached fairly quickly and the process cannot complete its tasks. This leads to an unknown error. If we check the platform first, we can call a sync version of the copy function, which is acceptable. In short, with this modification, my build works. Without it, I can't build when the number of build files exceeds a certain amount (which some do).