This patch adds a new compat_libevent.[ch] set of files, and moves our
Libevent compatibility and utilitity functions there. We build them
into a separate .a so that nothing else in src/commmon depends on
Libevent (partially fixing bug 507).
Also, do not use our own built-in evdns copy when we have Libevent
2.0, whose evdns is finally good enough (thus fixing Bug 920).
Now, when you call tor --digests, it dumps the SHA1 digest of each
source file that Tor was built with. We support both 'sha1sum' and
'openssl sha1'. If the user is building from a tarball and they
haven't edited anything, they don't need any program that calculates
SHA1. If they _have_ modified a file but they don't have a program to
calculate SHA1, we try to build so we do not output digests.
