This is required to be set and be equal to write size on FreeBSD.
    Note that despite documentation says it should be in host order,
    it only works if specified in network order

commit b551ed495bcc508e614fd14a25912301b1122a17
Author: Dmitry Marakasov <amdmi3@amdmi3.ru>
Date:   Fri Oct 7 17:32:44 2016 +0300

commit 4bc7d9cf77864ea32367cd80e37daa2ea1c630eb
Author: Dmitry Marakasov <amdmi3@amdmi3.ru>
Date:   Fri Oct 7 17:30:42 2016 +0300

commit e7e736d85ca80bb6e5b9c9694a1c002d1875b5c0
Author: Dmitry Marakasov <amdmi3@amdmi3.ru>
Date:   Fri Oct 7 17:22:59 2016 +0300

commit e55acfd1a11b2891e8bb467ac8e147ef58a82ce1
Author: Dmitry Marakasov <amdmi3@amdmi3.ru>
Date:   Fri Oct 7 17:22:12 2016 +0300

commit 8afbeacfce52d43b7ffb92eead2fccfab7f575ba
Author: Dmitry Marakasov <amdmi3@amdmi3.ru>
Date:   Fri Oct 7 17:19:05 2016 +0300

only available with uClibc if the C library is build with NPTL support (sets
__UCLIBC_HAS_THREADS_NATIVE__) and __UCLIBC_HAS_ADVANCED_REALTIME__ set.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>

Defining _GNU_SOURCE is required to get proper member names in `struct udphdr`
when building against musl. Build error:

  util.c: In function 'RawFillHeaders':
  util.c:481:9: error: 'struct udphdr' has no member named 'source'
       udph->source = htons(portsrc);
           ^
  util.c:482:9: error: 'struct udphdr' has no member named 'dest'
       udph->dest   = htons(portdst);
           ^
  util.c:483:9: error: 'struct udphdr' has no member named 'len'
       udph->len    = htons(sizeof(struct udphdr) + len);
           ^
  util.c:484:9: error: 'struct udphdr' has no member named 'check'
       udph->check  = 0;

Including <sys/types.h> in needed for mode_t, otherwise musl build fails:
  util.h:91:1: error: unknown type name 'mode_t'

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>

Otherwise building multicat in a static context fails:
  aggregartp.c:381: undefined reference to `pthread_self'

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>

Under Linux, the socket will receive all packets for a given multicast
address, from any interface, even if the interface address is specified.
SO_BINDTODEVICE requires the CAP_NET_ADMIN capability.

Patch courtesy of Benjamin Cohen bcohen AT openheadend DOT tv.

reopen everything).

by Benjamin Cohen.