On FreeBSD older than 9.1, it seems TCP_KEEPIDLE is not available and does not have a replacement ( https://lists.freebsd.org/pipermail/freebsd-net/2013-May/035496.html ). I don't know if this means anything for kfreebsd though.
One way of handling this is to wrap its use in #ifdef TCP_KEEPIDLE

diff --git a/server/reds.c b/server/reds.c
index 6064a6d..0c8fb35 100644
--- a/server/reds.c
+++ b/server/reds.c
@@ -2329,7 +2329,9 @@ static void reds_handle_ssl_accept(int fd, int event, void *data)
 static bool reds_init_keepalive(int socket)
 {
     int keepalive = 1;
+#ifdef TCP_KEEPIDLE
     int keepalive_timeout = KEEPALIVE_TIMEOUT;
+#endif
 
     if (setsockopt(socket, SOL_SOCKET, SO_KEEPALIVE, &keepalive, sizeof(keepalive)) == -1) {
         if (errno != ENOTSUP) {
@@ -2338,6 +2340,7 @@ static bool reds_init_keepalive(int socket)
         }
     }
 
+#ifdef TCP_KEEPIDLE
     if (setsockopt(socket, SOL_TCP, TCP_KEEPIDLE,
                    &keepalive_timeout, sizeof(keepalive_timeout)) == -1) {
         if (errno != ENOTSUP) {
@@ -2345,6 +2348,7 @@ static bool reds_init_keepalive(int socket)
             return false;
         }
     }
+#endif
 
     return true;
 }

I would add AC_CHECK_DECL for the symbol, so the info that it is unavailable is in the config log

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