Extract libevent in /var/tmp/dist rather than /var/tmp/build so that
the x86_64-apple-darwin10-install_name_tool command can work.

At the same time, we also change openssl to use this directory.

This problem was reported in this comment:
https://trac.torproject.org/projects/tor/ticket/17379#comment:12

This is building for Linux and Windows, but not yet for OSX.