[new] geo/librttopo

Hi,

here's a new port for librttopo, a new dependency for an upcoming
spatialite 5.0 update - nothing should pick it up besides spatialite
anyway.

The RT Topology Library exposes an API to create and manage standard
(ISO 13249 aka SQL/MM) topologies using user-provided data stores.

The code is derived from PostGIS liblwgeom library enhanced to provide
thread-safety, have less dependencies and be independent from PostGIS
release
cycles.

oks welcome!

Landry