diff --git a/cf/sunos.m4 b/cf/sunos.m4
index 57a244e90..938fc3a5e 100644
--- a/cf/sunos.m4
+++ b/cf/sunos.m4
@@ -11,6 +11,8 @@ case "$host" in
 *-*-solaris2.1[[1-9]])
 	AC_DEFINE([__EXTENSIONS__], [1],
 		  [Enable general extensions on Solaris.])
+	AC_DEFINE([_STDC_C11_BCI], [1],
+		[Enable C11 prototypes for memset_s and friends])
 	sunos=511
 	;;
 *-*-solaris2.[[89]] | *-*-solaris2.10)