[PROJ] Updating our C++ requirement to C++17 for PROJ 9.6

[Even Rouault]

Hi,

Happy New Year!

I propose we update our build requirement from C++11 to C++17. This 
should hopefully be unnoticed by anyone using a not too antiquated build 
chain.

Initial trigger for this move is explained in 
https://github.com/OSGeo/PROJ/pull/4366. All our existing CI 
configurations already satisfy the C++17 requirement (and one of them 
was already testing C++20 compatibility)

I don't anticipate much use of new capabilities for now except perhaps 
replacing our internal::make_unique<> by C++14 std::make_unique<>

C++17 has been a build requirement for GDAL since one year and nobody 
complained. Cf 
https://gdal.org/en/stable/development/rfc/rfc98_build_requirements_gdal_3_9.html 
for an analysis of the impacts.

This also satisfies https://proj.org/en/stable/community/rfc/rfc-3.html 
which mentions "Keeping a policy of always lagging behind be two 
iterations of the standard is thought to be the best comprise between 
the two concerns", given that C++20 and C++23 are out.

Even

-- 
http://www.spatialys.com
My software is free, but my time generally not.
Butcher of all kinds of standards, open or closed formats. At the end, this is just about bytes.