diff --git a/libratbox/configure.ac b/libratbox/configure.ac
index 86588c8..c6c40e5 100644
--- a/libratbox/configure.ac
+++ b/libratbox/configure.ac
@@ -80,7 +80,8 @@ case "$host_os" in
      CPPFLAGS="$CFLAGS -D_ALL_SOURCE -D_XOPEN_SOURCE=500"
      ;;
 *solaris*)
-     CPPFLAGS="$CFLAGS -D_POSIX_PTHREAD_SEMANTICS"
+*sunos*)
+     CPPFLAGS="$CFLAGS -D_POSIX_PTHREAD_SEMANTICS -D_XPG4_2 -D__EXTENSIONS__"
      ;;
 *)
      ;;