Scene::create_light_node<T> is defined only in scene.cpp but called
directly from blender/light.cpp and blender/geometry.cpp. Nothing
explicitly instantiates it, so those TUs rely on the implicit
instantiations in scene.cpp surviving as out-of-line COMDAT symbols.
With GCC at -O3 they are inlined into the create_node<T>
specializations and discarded, and the final link fails with
"undefined symbol: ccl::Scene::create_light_node<...>" for all five
light types. Add explicit instantiation definitions, which are
guaranteed to be emitted with external linkage.
--- a/intern/cycles/scene/scene.cpp
+++ b/intern/cycles/scene/scene.cpp
@@ -875,6 +875,14 @@ template<class T> T *Scene::create_light_node()
   return node_ptr;
 }

+/* Explicit instantiations for use from blender/light.cpp and
+ * blender/geometry.cpp. */
+template PointLight *Scene::create_light_node<PointLight>();
+template SpotLight *Scene::create_light_node<SpotLight>();
+template AreaLight *Scene::create_light_node<AreaLight>();
+template SunLight *Scene::create_light_node<SunLight>();
+template BackgroundLight *Scene::create_light_node<BackgroundLight>();
+
 template<> PointLight *Scene::create_node<PointLight>()
 {
   return create_light_node<PointLight>();