undef _FORTIFY_SOURCE before defining it, in case it's already set

git-svn-id: https://gnunet.org/svn/gnunet@33430 140774ce-b5e7-0310-ab8b-a85725594a96
social: separate host/guest/place API; add missing method cb params; improve docs

git-svn-id: https://gnunet.org/svn/gnunet@33431 140774ce-b5e7-0310-ab8b-a85725594a96