setup.py: ./configure and make libevent if libevent sources are found. link gevent/core.so statically against libevent.
If a directory "libevent-src" is found, we use that as default to the
--libevent argument.

If the --libevent directory does not contain a .libs directory (that
is if make hasn't been run), setup.py will now run configure and make
in order to link statically against libevent.

It also contains some workarounds to correctly build on OS X:
  - it does build a "fat" gevent.core
  - it adapts linker flags and uses the statically linked libevent
    even if a dynamic one is on the search path