aql: configure header for standard library files location

Let's start working on a standard library [1]. The idea is that we
bundle some aql files with predefined functions (and procedures) and we
load them when aql prompt starts.

So let's prepare a configuration file with a location for the scripts.
The location is currently only in local dev tree. We will later change
that to some /usr/share or /usr/lib location.

[1] #221