fix building on osx

    On Linux, they use 'BIND_4_COMPAT' and #include <arpa/nameser_compat.h> in <arpa/nameser.h>
    On OSX, if 'BIND_8_COMPAT' is #defined, <arpa/nameser_compat.h> is #included in <arpa/nameser.h>
Merge pull request #18 from zyxar/master

fix building on osx