[Freeciv-Dev] (PR#5122) Building requirement for techs

["Guest" <rt-guest@xxxxxxxxxxxxxx>]

This patch adds new fields in techs.ruleset:
- bldg_req: At least one city must have this building to develop the
technology
- bldg_req_penalty: Cities that don't have this building lose AMOUNT
percent of research when the player is researching the tech.

There is still some problems (Ai doesn't understand very well, ruleset
checking, possibility of no more tech available) but I send it to have
your opinion about this feature.

I think that it can be useful in two ways:
- no more smallpox (with the penalty, there will be a need for too many
improvements to have a correct research) 
- modpacks (in Middle-Earth, reserching Nine for The Men will require
The One Ring building)

tech_bldg_req1.diff
Description: tech_bldg_req1.diff