[Qgis-developer] Requiring c++17 minimum

[Nyall Dawson]

Hi all,

I read on phoronix this morning that c++17 is nearly ready. I think
it's time we made this the minimum build dependency for QGIS (3.0 and
2.18.5, and maybe 2.14 (but not sure about 2.14 since it's the LTS -
can someone file a QEP?)).

Advantages:
- has some neat stuff like parallel algorithms
- can use std::variant instead of qvariant and std::optional instead
of QgsOptional
- 17 is bigger than 11

Disadvantages:
- I think we lose the Windows builds. That's a shame, but we need to
move forward.
- OSX users will need to build their own version of clang/llvm from
git. That's ok - they are used to things that "just work" so I think
they'll quickly adapt to this process.
- Linux users will need to update to the next beta version of their
distro. This is just a short-term inconvenience and I'm pretty sure
no-one will lose productivity because of it.

I'm certain this is the way forward, so I'll push a change later today
requiring this.

Nyall