Do not add epoll_sub (syscall wrappers) for epoll in cmake

Do not add epoll_sub (syscall wrappers) for epoll in cmake

Nowadays mostly all supported OS'es has this wrappers, and some of them (like
SmartOS) has wrappers but doesn't have __NR_epoll* defines for syscall numbers,
so just drop them (instead of adding yet another check int cmake like autotools
has, since this will break building in cross-compile environment).

Also one minor note, configure doesn't added epoll_sub.c either, since it check
epoll_create() in runtime.

And I tested it in SmartOS and it even works (`make verify`).

Fixes: #463