If using external png need to link vtkpython with it.

Change-Id: I8ea03dd6bcad307ac52ca9d64fe67a20c8a8e5cd

Wrapping/Python/CMakeLists.txt

@@ -280,6 +280,10 @@ IF(VTK_USE_FFMPEG_ENCODER)
   LIST(APPEND VTKPYTHON_LINK_LIBS ${FFMPEG_BASIC_LIBRARIES})
 ENDIF(VTK_USE_FFMPEG_ENCODER)
 
+IF(VTK_USE_SYSTEM_PNG)
+  LIST(APPEND VTKPYTHON_LINK_LIBS ${PNG_LIBRARY})
+ENDIF(VTK_USE_SYSTEM_PNG)
+
 TARGET_LINK_LIBRARIES(vtkpython ${VTKPYTHON_LINK_LIBS})
 TARGET_LINK_LIBRARIES(vtkpython LINK_INTERFACE_LIBRARIES ${VTK_PYTHON_LIBRARIES})
 
@@ -295,7 +299,7 @@ ENDIF(VTK_USE_MATLAB_MEX)
 SET(PVTKPYTHON_EXECUTABLE)
 # Create the pvtkpython Python wrapper executable with MPI support.
 IF (VTK_USE_PARALLEL AND VTK_USE_MPI)
-  INCLUDE_DIRECTORIES("${MPI_INCLUDE_PATH}")
+  INCLUDE_DIRECTORIES(${MPI_INCLUDE_PATH})
   IF (MPI_LIBRARY)
     SET(MPI_LIBRARIES ${MPI_LIBRARY})
   ENDIF (MPI_LIBRARY)