If the PROJ_DB_CACHE_DIR environment variable is defined, then a
$(PROJ_DB_CACHED_DIR)/proj.db.sql.md5 file is used to determine if
the set of .sql files has changed since the last time. If not then
$(PROJ_DB_CACHED_DIR)/proj.db is directly used.
This can saved a few seconds when doing rebuilds.
This is a poor man equivalent of ccache for generating the database :-)

Latest breathe 4.13.0 no longer work with sphinx 1.8.5 / Pyhon 2,
so force use 4.12.0 for now.

See
https://travis-ci.com/OSGeo/proj.4/jobs/194629602
https://github.com/michaeljones/breathe/issues/431

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

All the GIGS JSON files have been converted to the gie format
and are now covered by running "make check". Therefore we no
longer need the Python setup. Removing it from the Travis CI
setup should speed up the test execution time a bit.

The multistresstest code has been made C89 compliant in the process.