fix: define PYBIND11_CPP14 for recent intel compilers

[tobiasleibner]

I just encountered #1649 with the Intel compiler 2021 (beta). Recent Intel compilers seem to support C++14 just fine, so just defining PYBIND11_CPP14 for recent Intel compilers should fix the problem.

EDIT (@YannickJadoul): Closes #1649

Suggested changelog entry:

* Intel compiler was not activating C++14 support due to a broken define.

[henryiii]

Is it really only added on 19? We only support 18+, so it's much simpler if it was added in 18. :'( I don't see anything specific in the release notes for 19 that indicate that they just started defining __cplusplus.

[tobiasleibner]

I guess __cplusplus is already defined for 18, but the first comment in #1649 mentions that the C++14 support is not sufficient in that version. I will check the value of __cplusplus in version 18.

[tobiasleibner]

Apparently, __cplusplus is already defined to 201402L in version 18. So it should work in theory, but apparently it does not work in practice, see #1649. To support version 18, we thus would have to add specializations to work around the intel compiler bugs, I guess.

[henryiii]

This would be better than forcing C++ mode to 11 if 14 is requested. It would even be better, IMO, to throw an error if 14 is requested instead of using 11 when 14 is set, with a statement like "only C++11 is supported with Intel 18". I could make a quick attempt to get this working on Intel 18 before doing something like that.

It would be so much nicer if we could test this in CI, but only the most recent version is available and it doesn't work (#2573 )

[YannickJadoul]

I'm not a fan of adding workarounds to support a compiler lying about supporting C++14, though :-/

[henryiii]

At the minimum, let's kill the Intel specific limiting here. Since we already have a list of #error printouts for minimum supported compilers, we could add on that forces Intel 18 to C++11 mode.

[tobiasleibner]

I removed the first check and added an error message.