Added friend class PerContextProgram;

include/osg/Program

@@ -386,6 +386,7 @@ class SG_EXPORT Program : public osg::StateAttribute
 
     protected:
 	/** PerContextProgram (PCP) is an OSG-internal encapsulation of glPrograms per-GL context.  */
+
 	class PerContextProgram : public osg::Referenced
 	{
 	    public:
@@ -429,6 +430,10 @@ class SG_EXPORT Program : public osg::StateAttribute
 		PerContextProgram& operator=(const PerContextProgram&);	// disallowed
 	};
 
+        // make PerContextProgram a friend to allow it access Program's protected
+        // methods and member variables.
+        friend class PerContextProgram;
+
     protected:	/*methods*/
         virtual ~Program();
 	/** Get the PCP for a particular GL context */