Add "include" option for config file (try 2).

I actually did some testing of my last attempt at this, but I'm not sure
in retrospect how kdesrc-build even got something coherent from my first
patch given that I forgot to actually assign the output of the various
read line functions to $_ (which had previously been automatically
done), or how I passed the wrong object type to read_line (now
readLogicalLine).

I've increased the testing performed on the patch and it should actually
work this time. For serious.

BUG:223331