Avoid copying list of links by maintaining a 'dirty' flag that causes
_notify_links to restart iteration. This makes sure that we don't miss
a recently added link because notification is already in progress.

It is possible to notify the same links (that did not remove themselvers)
more than once, but it's better than not notifying some links at all.

rawlink() no longer schedules notification immediatelly if needed, because
there seem to be utility in such behaviour.

That way generic pool methods are grouped together.

without these checks passing invalid arguments would crash the process;
with the checks it raises ValueError.

Based on http://git.42mm.org/?p=python-gevent;a=blob;f=debian/patches/setup-with-cares-and-libev;h=798af501694f1d12cfdaf817bb2e7ad2f07bb2a0;hb=HEAD

http://git.42mm.org/?p=python-gevent;a=blob;f=debian/patches/do-not-include-ares-setup;h=28b1e91eba74da4bae913e99f6348d6c38f468eb;hb=HEAD

xcode is based on llvm-gcc, which btw is deprecated. The compilation
of libev bails out with:

,----
| libev/ev.c:885:3: error: #error "memory fences not defined for your architecture, please report"
`----

We work around it by undefining __llvm__ which makes ev.c trust
gcc-llvm's advertised gcc version.

See http://lists.schmorp.de/pipermail/libev/2011q4/001618.html for a
discussion of the issues involved.

to be used when passing events integer as first argument is desirable
the old method of passing EVENTS object as first argument is still
used internally but no longer advised or advertised