This work mostly consists of:
- a C++ implementation of the ISO-19111:2018 / OGC Topic 2
  "Referencing by coordinates" classes to represent Datums,
  Coordinate systems, CRSs (Coordinate Reference Systems) and
  Coordinate Operations.
- methods to convert between this C++ modeling and WKT1, WKT2
  and PROJ string representations of those objects
- management and query of a SQLite3 database of CRS and Coordinate Operation definition
- a C API binding part of those capabilities

This is all-in-one squashed commit of the work of
https://github.com/OSGeo/proj.4/pull/1040

These functions aren't called from anywhere else in the codebase.

CMake provides this function, but only in CMake 2.8.3 and up. This
minimum version bump is OK because it is from 2010, but the code
requires C++11 features to exist. Any platform with a suitable compiler,
but too-old of a CMake can easily compile the required CMake version as
well.