Update compile flag to c++14 (or c++17)

[jpconstantineau]

Is your feature request related to a problem? Please describe.
I want to add a c++ library that depends on C++14 features. Upgrading from c++11 to c++14 should take care of that.

Describe the solution you'd like
Update the platform.txt file to update the gcc command like flags.
https://github.com/adafruit/Adafruit_nRF52_Arduino/blob/master/platform.txt#L43

Describe alternatives you've considered
I have tested it here locally on a fork and haven't seen any issues (c++14 should be backward compatible with c++11).

Additional context
If you are OK with going ahead, I can make the change here and create a PR. I just want to know if you would approve the update as it could impact other things I may not be aware of. The gcc arm compiler has been supporting this flag for quite a while.

[jpconstantineau]

This would be done similarly to this PR: SpenceKonde/ATTinyCore@d5c5cf6

[hathach]

are you also using gcc 9 as the bsp. If you feel free to submit an PR, however, it may not be merged immediately. There is a quite pending number of issues I would like to resolve first and changing compiler dialect could complicate the issue.

[jpconstantineau]

That probably wise. Also, I would probably recommend taking the smaller step of going to 14 first as many people indicate that mostly a "bug fix" of c++11. c++17 brought more things... There are even talks about some gotchas of moving to c++17. https://youtu.be/mAZyaAo3M70

[jpconstantineau]

@hathach I just started testing 0.22.1 with my project and quite a few errors are coming out. Some from code that hasn't changed in years. Other places is new with the 0.22.x releases.