configure: netinet/in_pcb.h and netinet/ip_var.h require netinet/in.h on Mac OS X.
diff --git a/configure.ac b/configure.ac
index 199e2bb..55c6562 100644
--- a/configure.ac
+++ b/configure.ac
@@ -284,6 +284,9 @@
      #endif
      #if HAVE_SYS_SOCKETVAR_H
      # include <sys/socketvar.h>
+     #endif
+     #if HAVE_NETINET_IN_H
+     # include <netinet/in.h>
      #endif])
 
 AC_CHECK_HEADERS([netinet/tcp_var.h],,,