Make gcc warnings on by default, and --enable-gcc-warnings only add -Werror This commit is based on a patch for Tor (git commit ca60a6ce3f4786626ac455ec1b798b2e8304635c by Peter Palfrader), Copyright (c) 2007-2011, The Tor Project, Inc. (Originally, it added --enable-gcc-warnings-advisory as in Tor; Nick changed that.)