Rational behind -std=gnu++14, and change it to -std=c++14

[mratsim]

Since Nim 1.2, Nim is using the -std=gnu++14 to compile C++ code.

What's the rationale behind it? Could -std=c++14 achieve the same goal?

Currently Arraymancer Cuda backend is only usable with Nim 1.0 due to that as there is no way to remove that options apart from override the user nim.cfg file. mratsim/Arraymancer#478

#13798 does not help, the flag must be absent from the command line.

[timotheecour]

@mratsim I was also wondering about this recently; can you try a PR that changes it to -std=c++14 and see what breaks?

[Araq]

We use GNU extensions like ranges inside switch and computed gotos.