Looking for a Linux threadpool api with OS scheduler support
You really can't do this without OS support. There's no good way to tell that a thread is blocked on I/O. You wind up having to atomically increment a counter before each operation that might block and decrement it after. Then you need a thread to monitor that counter and create an additional thread if it's above zero. (Remove threads if they're idle more than a second or so.)
Generally speaking, it's not worth the effort. This only works so well on Windows because it's the "Windows way" and Windows is built from the ground up for it. For Linux, you should be using epoll or boost::asio. Use something that does things the "Linux way" rather than trying to make the Windows way work on non-Windows operating systems.
You can write your own wrappers that use IOCP on Windows, epoll on Linux, and so on. But these already exist, so you need not bother.