Update to the new (version 5) PROJ.4 API

[ckhroulev]

See https://proj4.org/development/migration.html

[ckhroulev]

See OSGeo/PROJ#836

In the meantime we need to add

#define ACCEPT_USE_OF_DEPRECATED_PROJ_API_H

to src/util/Proj.hh.

[citibeth]

@ckhroulev @adamjstewart Do you know if any released versions of PISM require or allow PROJ.6? Or do all released versions still need to work with PROJ.4?

[ckhroulev]

@citibeth Thanks for the nudge! Please see https://github.com/pism/pism/releases/tag/v1.1.4

I will switch to PROJ v6 when I get a chance. (I updated the code but had to put it on the back burner for now since I have more pressing tasks to work on.)

[ckhroulev]

Done in the dev branch and will be a part of the next release.