Fixed a define for __sgi. Irix does not allow dlsym(0L, ...) without

calling dlopen() first, just like __sun

include/osg/GLExtensions

@@ -61,7 +61,7 @@ inline void* getGLExtensionFuncPtr(const char *funcName)
     } else
 	return NULL;
 
-#elif defined (__sun)
+#elif defined (__sun) || defined (__sgi)
 
      static void *handle = dlopen((const char *)0L, RTLD_LAZY);
      return dlsym(handle, funcName);