add option to use system GLEW library (MYGUI_USE_SYSTEM_GLEW)

Could be written shorter (and with no code duplication) like this:

<pre>
target_link_libraries(${PROJECTNAME} ${OPENGL_gl_LIBRARY} ${PNG_LIBRARIES} ${ZLIB_LIBRARIES} MyGUIEngine)
if(MYGUI_USE_SYSTEM_GLEW)
   target_link_libraries(${PROJECTNAME} ${GLEW_LIBRARIES})
endif()
</pre>