[Proj] Use of SQLite

[Greg Troxel]

> For the sake of avoiding to have different proj flavours, I'd be
> inclined to make sqlite a required dependency of proj. SQLite is
> available anywhere, so this shouldn't cause practical issues (binary
> distributions of the GIS stack have all sqlite packaged, being a
> requirement of GDAL, QGIS, spatialite, etc...). The SQLite
> capabilities and API effectively needed shouldn't require imposing a
> version newer than 3.7.0.

Agreed on making it mandatory - having it optional seems like a lot of
pain for no benefit.

3.7.0 sounds old enough that it's fair to tell anyone with something
older than that that they have to deal, even without understanding their
situation.