[Freeciv] Can't build Settlers yet.

[Matt Armstrong <matt@xxxxxxxxxx>]

Taking the advice of some of the strategy pages, I try to expand as
fast as possible in the early stages by building nothing but Settlers.
But for some cities I get a "Can't build Settlers yet" message when a city
is done building its settler.  I can't find a documented prerequisite
for Settlers, so what can I do to to allow these cities to build thier
Settlers?