Define IPV6_ADD_MEMBERSHIP for OSX

Travis error:

20:43:59 runner ['clang++', '-flax-vector-conversions', '-Wall', '-std=gnu++11', '-fPIC', '-Imacosx', '-I../macosx', '-Imacosx/RPC', '-I../macosx/RPC', '-Iposix', '-I../posix', '-Icommon', '-I../common', '-Icommon/jack', '-I../common/jack', '-I.', '-I..', '-I/usr/local/Cellar/opus/1.3/include/opus', '-DAFTEN=1', '-DHAVE_DOXYGEN=0', '-DHAVE_ALSA=0', '-DHAVE_FIREWIRE=0', '-DHAVE_IIO=0', '-DHAVE_PORTAUDIO=0', '-DHAVE_WINMME=0', '-DHAVE_CELT_API_0_11=0', '-DHAVE_CELT_API_0_8=0', '-DHAVE_CELT_API_0_7=0', '-DHAVE_CELT_API_0_5=0', '-DHAVE_CELT=0', '-DHAVE_OPUS_PKG=1', '-DHAVE_OPUS=0', '-DHAVE_SAMPLERATE=1', '-DHAVE_SNDFILE=1', '-DHAVE_STDIO_H=1', '-DHAVE_READLINE=1', '-DHAVE_SYSTEMD=0', '-DHAVE_DB_H=1', '-DHAVE_DB=0', '-DHAVE_ALLOCA_H=1', '-DHAVE_EXECINFO_H=1', '-DUSE_CLASSIC_AUTOLAUNCH=1', '-DCLIENT_NUM=64', '-DPORT_NUM_FOR_CLIENT=768', '-DADDON_DIR="/usr/local/lib/jack"', '-DJACK_LOCATION="/usr/local/bin"', '-DUSE_POSIX_SHM=1', '-DJACKMP=1', '-DHAVE_CONFIG_H', '-DSERVER_SIDE', '../posix/JackPosixThread.cpp', '-c', '-o/Users/travis/build/jackaudio/jack2/build/posix/JackPosixThread.cpp.2.o', '-fvisibility=hidden']
../posix/JackNetUnixSocket.cpp:455:26: error: use of undeclared identifier 'IPV6_ADD_MEMBERSHIP'
                option = IPV6_ADD_MEMBERSHIP;
                         ^
1 error generated.
Waf: Leaving directory `/Users/travis/build/jackaudio/jack2/build'
Build failed

posix/JackNetUnixSocket.cpp

@@ -27,6 +27,14 @@ Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 #include <unistd.h>
 #include <fcntl.h>
 
+// define IPV6_ADD_MEMBERSHIP for Mac OS X
+#ifndef IPV6_ADD_MEMBERSHIP
+#define IPV6_ADD_MEMBERSHIP IPV6_JOIN_GROUP
+#endif
+#ifndef IPV6_DROP_MEMBERSHIP
+#define IPV6_DROP_MEMBERSHIP IPV6_LEAVE_GROUP
+#endif
+
 using namespace std;
 
 // See RFC 3493; The Open Group Base Specifications Issue 6 IEEE Std 1003.1, 2004 Edition