private mathhub no library

continuing from #9: 
When installed, http://localhost/#/library gives me `Failed to load library`. 
And this is not surprising, since it does not know about a library yet. 
As most of the users of private MathHubs  have a previously installed local MathHub, `./configure` should ask for that (probably pick it up from a pre-existing MMT system implementation) and use that. 
 