Revert out extraneous changes.
Provide support for platforms without backtrace support (e.g. FreeBSD).
Produce an error message when ./configure is run if neither epoll or kqueue is supported, instead of producing confusing link errors because both nc_epoll.c and nc_kqueue.c are empty.

Conflicts:
	configure.ac

Conflicts:
	configure.ac
Remove extraneous newline.