Make use of SO_REUSEPORT conditional

Fixes: #244

SO_RESUEPORT was introduced in Linux 3.9 and later.
This change allows NCCL to compile against older releases.

The functionality is only required if the user is specifying
a NCCL bootstrap address via an environment variable.

src/include/socket.h

@@ -327,7 +327,11 @@ static ncclResult_t createListenSocket(int *fd, union socketAddress *localAddr)
   if (socketToPort(&localAddr->sa)) {
     // Port is forced by env. Make sure we get the port.
     int opt = 1;
+#if defined(SO_REUSEPORT)
     SYSCHECK(setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR | SO_REUSEPORT, &opt, sizeof(opt)), "setsockopt");
+#else
+    SYSCHECK(setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt)), "setsockopt");
+#endif
   }
 
   // localAddr port should be 0 (Any port)