configure: Set _DARWIN_C_SOURCE while testing for struct ipv6_mreq

On OS X, this struct is only available if _DARWIN_C_SOURCE is defined,
when we have _POSIX_C_SOURCE defined. This allows the struct to be found,
enabling proper IPv6 multicast functionality on OS X.

The define is already set within the file that uses the struct. Setting it
only for this test in configure avoids having to keep it defined for the
whole build.

Originally committed as revision 25370 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2794,7 +2794,7 @@ texi2html -version > /dev/null 2>&1 && enable texi2html || disable texi2html
 if enabled network; then
     check_type "sys/types.h sys/socket.h" socklen_t
     check_type netdb.h "struct addrinfo"
-    check_type netinet/in.h "struct ipv6_mreq"
+    check_type netinet/in.h "struct ipv6_mreq" -D_DARWIN_C_SOURCE
     check_type netinet/in.h "struct sockaddr_in6"
     check_type "sys/types.h sys/socket.h" "struct sockaddr_storage"
     check_struct "sys/types.h sys/socket.h" "struct sockaddr" sa_len