Support building on OS X

Support building on OS X

* Add support for new --without-glib configure option to disable
  all use of glib (also requires --without-gtk to be effective).
  This has the side effect of disabling the "mtr -z" functionality.

* Improve configure support for detecting which library to link
  with, if any, to get the resolver functions.

* Do not define BIND_8_COMPAT when compiling for Darwin/OS X.

* Autoconfigure whether or not a suitable struct __res_state_ext
  needs to be defined.

* If the arpa/nameser_compat.h header is available and usable,
  include it immediately after arpa/nameser.h.