checking whether CONNECT_RESUME_ON_READ_WRITE is declared... no
configure: error: TCP Fast Open is not available for client mode: please rerun without --enable-tfo-client

--- a/configure.ac	2020-12-03 09:31:18.000000000 +0100
+++ b/configure.ac	2021-01-26 01:16:58.000000000 +0100
@@ -120,6 +120,9 @@
 AC_SUBST(LIBUNBOUND_REVISION)
 AC_SUBST(LIBUNBOUND_AGE)
 
+# AC_CANONICAL_HOST is needed to access the 'host_os' variable
+AC_CANONICAL_HOST
+
 
 cmdln="`echo $@ | sed -e 's/\\\\/\\\\\\\\/g' | sed -e 's/"/\\\\"/'g`"
 AC_DEFINE_UNQUOTED(CONFCMDLINE, ["$cmdln"], [Command line arguments used with configure])
@@ -1226,15 +1229,15 @@
 AC_ARG_ENABLE(tfo-client, AC_HELP_STRING([--enable-tfo-client], [Enable TCP Fast Open for client mode]))
 case "$enable_tfo_client" in
 	yes)
-		case `uname` in
-			Linux) AC_CHECK_DECL([MSG_FASTOPEN], [AC_MSG_WARN([Check the platform specific TFO kernel parameters are correctly configured to support client mode TFO])],
+		case "${host_os}" in
+			linux*) AC_CHECK_DECL([MSG_FASTOPEN], [AC_MSG_WARN([Check the platform specific TFO kernel parameters are correctly configured to support client mode TFO])],
 			                     [AC_MSG_ERROR([TCP Fast Open is not available for client mode: please rerun without --enable-tfo-client])], 
 			                     [AC_INCLUDES_DEFAULT 
 #include <netinet/tcp.h>
 ])
 					AC_DEFINE_UNQUOTED([USE_MSG_FASTOPEN], [1], [Define this to enable client TCP Fast Open.])
 			  ;;
-			Darwin) AC_CHECK_DECL([CONNECT_RESUME_ON_READ_WRITE], [AC_MSG_WARN([Check the platform specific TFO kernel parameters are correctly configured to support client mode TFO])], 
+			darwin*) AC_CHECK_DECL([CONNECT_RESUME_ON_READ_WRITE], [AC_MSG_WARN([Check the platform specific TFO kernel parameters are correctly configured to support client mode TFO])], 
 			                      [AC_MSG_ERROR([TCP Fast Open is not available for client mode: please rerun without --enable-tfo-client])], 
 			                      [AC_INCLUDES_DEFAULT
 #include <sys/socket.h>