And buildconf/nb_procs to buildconf/num_procs.

Add some indentation to make things more clear.

We use the new compiler Mozilla is using for Firefox 52 cross-builds as
well. clang now requires GCC 4.8.5 for compiling which is not available
in Debian Wheezy. We therefore build the macOS parts on Debian Jessie
now.

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.