* Makefile.in: new variable PTHREAD_DEFS for portability

    * Makefile.in: new variable PTHREAD_DEFS for portability
        * aidepost.c, citadel.c, citmail.c, mailinglist.c, msgform.c,
          netmailer.c, netpoll.c, netproc.c, rcit.c, readlog.c, setup.c,
          stats.c, userlist.c, whobbs.c: return type of main() is int
        * citadel.c, commands.c, messages.c: use time_t properly
        * citserver.c: include <limits.h>