Add io_uring detection support to CMakeLists.txt

[xorangekiller]

io_uring support was added to libev after I did the initial build. It is supported in the current version of libev in master, but CMake doesn't detect it. The definitions HAVE_LINUX_FS_H && HAVE_SYS_TIMERFD_H && HAVE_KERNEL_RWF_T are checked for in ev.c, and would need to be detected in CMakeLists.txt and added to config.h.cmakein.

It may be worth going through the other HAVE_ definitions in that file to ensure that all options are correctly detected and supported in the CMake build compared to the upstream autotools build. ev.pod has a list of those options with descriptions.

This issue is up for grabs. If you need io_uring support, a contribution would be welcome. I'm not actively using libev anymore, so I likely won't add it myself.