RHEL8 compatibility

[Jason A. Donenfeld]

Does something like this work?

diff --git a/src/compat/compat.h b/src/compat/compat.h
index 54907e9a..888974a3 100644
--- a/src/compat/compat.h
+++ b/src/compat/compat.h
@@ -90,7 +90,7 @@
     (LINUX_VERSION_CODE < KERNEL_VERSION(3, 18, 27) &&
LINUX_VERSION_CODE >= KERNEL_VERSION(3, 17, 0)) || \
     (LINUX_VERSION_CODE < KERNEL_VERSION(3, 16, 8) &&
LINUX_VERSION_CODE >= KERNEL_VERSION(3, 15, 0)) || \
     (LINUX_VERSION_CODE < KERNEL_VERSION(3, 14, 40) &&
LINUX_VERSION_CODE >= KERNEL_VERSION(3, 13, 0)) || \
-    (LINUX_VERSION_CODE < KERNEL_VERSION(3, 12, 54))) && !defined(ISUBUNTU1404)
+    (LINUX_VERSION_CODE < KERNEL_VERSION(3, 12, 54))) &&
!defined(ISUBUNTU1404) && !defined(ISRHEL7)
 #include <linux/if.h>
 #include <net/ip_tunnels.h>
 #define IP6_ECN_set_ce(a, b) IP6_ECN_set_ce(b)