http://bugs.gentoo.org/show_bug.cgi?id=366073

Patch written by Nils Larsson <ni1s@nerdshack.com>
--- src/celengine/frametree.h
+++ src/celengine/frametree.h
@@ -14,6 +14,7 @@
 #define _CELENGINE_FRAMETREE_H_
 
 #include <vector>
+#include <cstddef>
 
 class Star;
 class Body;