Sources here are from luasql git tag 2.6.0
(https://github.com/lunarmodules/luasql/tree/2.6.0)
Some modifications have been done to them, but as of 08-Oct-22
all our modifications have been accepted to the upstream.

Only the files needed by freeciv are included here, not entire luasql
source directory hierarchy.
The ls_*.h files are our own additions to use those backends directly.
