Only in patched/gazebo-9.0.0: build
diff -ura original/gazebo-9.0.0/gazebo/rendering/CustomPSSMShadowCameraSetup.cc patched/gazebo-9.0.0/gazebo/rendering/CustomPSSMShadowCameraSetup.cc
--- original/gazebo-9.0.0/gazebo/rendering/CustomPSSMShadowCameraSetup.cc	2018-01-25 23:25:48.000000000 +0100
+++ patched/gazebo-9.0.0/gazebo/rendering/CustomPSSMShadowCameraSetup.cc	2018-02-24 12:29:12.219115580 +0100
@@ -49,6 +49,8 @@
 
 #include "gazebo/rendering/CustomPSSMShadowCameraSetup.hh"
 #include "gazebo/rendering/ogre_gazebo.h"
+#include <OGRE/RTShaderSystem/OgreShaderFunction.h>
+#include <OGRE/RTShaderSystem/OgreShaderProgram.h>
 
 using namespace gazebo;
 using namespace rendering;