diff --git a/sys/conf/kern.mk b/sys/conf/kern.mk
--- a/sys/conf/kern.mk
+++ b/sys/conf/kern.mk
@@ -231,6 +231,16 @@
 #
 CFLAGS+=	-fwrapv
 
+#
+# Enabling the MS extensions allows us to use a non-C-standard feature
+# of embedding an anonymous struct in another.
+# The compilers supported (-fms-extensions) at least since:
+# - GCC for C since 750491fc108a (releases/gcc-3.3.0),
+# - clang since 1f53eb6fbd71b (llvmorg-3.8.0).
+#
+CFLAGS+=	-fms-extensions
+CFLAGS.clang+=	-Wno-microsoft-anon-tag
+
 #
 # Stack Smashing Protection (SSP) support
 #