Switch to C++17

[jcoupey]

Upgrading to C++17 would probably not be a huge change for the codebase. The only drawback I can see would be to drop support for some of the older compilers but we can definitely live with that.

On the other hand, some of the new language features like std::optional could help us reduce dependencies requirements. The impact on building Docker images or launching CI builds is probably worth the trouble.

[jcoupey]

One last problem is that when building against libosrm the -std=c++14 flag is retrieved along with added flags when running:

 pkg-config --cflags libosrm

So we end up with two conflicting values.