[syslog-ng] 1.9.11 compile failure on IRIX 6.5, Tru64 UNIX 5.1

[Albert Chin]

Seems like IRIX 6.5 and Tru64 UNIX 5.1 don't have IPV6_ADD_MEMBERSHIP.
They do have IP_ADD_MEMBERSHIP though. Should we just:
  #ifndef IPV6_ADD_MEMBERSHIP
  #define IPV6_ADD_MEMBERSHIP IP_ADD_MEMBERSHIP
  #endif

cc -DHAVE_CONFIG_H -I. -I. -I..
-DPATH_SYSCONFDIR=\"/etc/opt/TWWfsw/syslogng1911\"
-DPATH_LOCALSTATEDIR=\"/var/opt/TWWfsw/syslogng1911\"
-I/opt/TWWfsw/libglib26/include -I/opt/TWWfsw/libglib26/lib/include
-I/opt/TWWfsw/gettext014/include
-I/opt/TWWfsw/syslogng198/include/eventlog   -D_GNU_SOURCE
-Wl,-woff,84 -Wl,-woff,85 -woff 1009,1429 -use_readonly_const -G0
-rdata_shared -DEBUG:optimize_space=ON -O2 -OPT:Olimit=0 -c afinet.c
cc-1020 cc: ERROR File = afinet.c, Line = 190
  The identifier "IPV6_ADD_MEMBERSHIP" is undefined.

                  setsockopt(fd, IPPROTO_IPV6, IPV6_ADD_MEMBERSHIP,
&mreq6, sizeof(mreq6));
                                               ^

1 error detected in the compilation of "afinet.c".

-- 
albert chin (china at thewrittenword.com)