unix/Makefile: Allow users to specify the C++ standard used.

[stinos]

Mainly for user modules.

Actually, I'm not sure the whole HASCPP17 part should even be there: it's not needed for the MicroPython source itself (that just needs at least c++11 because the usercmodule example uses that). I'd be happy to remove it, but it's maybe too late for that now (as in: people relying on it)?

[dpgeorge]

that just needs at least c++11 because the usercmodule example uses that

Maybe that usercmodule can update the flags in its .mk file?

but it's maybe too late for that now (as in: people relying on it)

I don't think they would be. And if they are it's an easy fix on the command line to append to CFLAGS_EXTRA.

So I'd say remove the HASCCP17 thing.

[stinos]

Maybe that usercmodule can update the flags in its .mk file?

That probably makes most sense, and then it can be removed from the main Makefile completely. However C++14 has actually been the default for gcc since v6 which is about 6 years old by now. So simply removing, without any addition to the .mk, is actually going to work for a lot of cases. stm32 port does that as well btw. Ok?

[dpgeorge]

So simply removing, without any addition to the .mk, is actually going to work for a lot of cases. stm32 port does that as well btw. Ok?

Yes that sounds OK. As long as the CI passes 😄

[stinos]

As long as the CI passes

Was going to say 'yeah sure' but then the macos build failed, so added in mpconfigport.mk after all.

[dpgeorge]

so added in mpconfigport.mk after all.

That's fine, and it adds more good content to the example anyway, to show other people how to add options there.

Merged in 8bb50c6 and c14f47f