In CMake builds, link glgsg with CG_LIBRARIES.

panda/src/glgsg/CMakeLists.txt

@@ -10,6 +10,7 @@ if(HAVE_GL)
 
   composite_sources(p3glgsg P3GLGSG_SOURCES)
   add_library(p3glgsg ${P3GLGSG_HEADERS} ${P3GLGSG_SOURCES})
-  target_link_libraries(p3glgsg p3glstuff p3display ${OPENGL_LIBRARIES})
+  target_link_libraries(p3glgsg p3glstuff p3display
+    ${OPENGL_LIBRARIES} ${CG_LIBRARIES})
   target_interrogate(p3glgsg ALL)
 endif()