Assuming Libevent has been installed on Mac, and now try to compile something that uses it such as the toy low-level ROT13 server with Libevent:

$ gcc libeventBasedRot13Server.c

would failed with something like:

Undefined symbols:
  "_evutil_make_socket_nonblocking", referenced from:
      _do_accept in cc2B6M02.o
      _run in cc2B6M02.o
  "_event_new", referenced from:
      _alloc_fd_state in cc2B6M02.o
      _alloc_fd_state in cc2B6M02.o
      _run in cc2B6M02.o
  "_event_add", referenced from:
      _do_read in cc2B6M02.o
      _do_accept in cc2B6M02.o
      _run in cc2B6M02.o
  "_event_base_dispatch", referenced from:
      _run in cc2B6M02.o
  "_event_free", referenced from:
      _alloc_fd_state in cc2B6M02.o
      _free_fd_state in cc2B6M02.o
      _free_fd_state in cc2B6M02.o
  "_event_base_new", referenced from:
      _run in cc2B6M02.o
  "_event_del", referenced from:
      _do_write in cc2B6M02.o
ld: symbol(s) not found
collect2: ld returned 1 exit status

Solution:

$ gcc libeventBasedRot13Server.c /usr/local/lib/libevent.a