v9.11 fails to build on macOS

Hello. While packaging v9.11 of OpenConnect for Homebrew we are encountering a build failure. The error we are receiving is below.

  os-tcp-mtu.c:126:12: error: use of undeclared identifier 'IP_PMTUDISC_DO'
          int val = IP_PMTUDISC_DO;
                    ^
  os-tcp-mtu.c:127:38: error: use of undeclared identifier 'IP_MTU_DISCOVER'
          ret = setsockopt(sock, IPPROTO_TCP, IP_MTU_DISCOVER, &val, sizeof(val));
                                              ^
  2 errors generated.
  make[1]: *** [os-tcp-mtu.o] Error 1

Relates to https://github.com/Homebrew/homebrew-core/pull/131205