configure check for -std=c++17 insufficient

[mkoeppe]

Describe the bug

debian-stretch has gcc 6.3.0-18+deb9u1, which accepts the flag -std=c++17 but does not have <optional>

https://trac.sagemath.org/ticket/34081#comment:34

[Mic92]

Where do they put <optional> on debian? Does this include more or less all standard c++ headers?

[Mic92]

Also is debian 9 not end-of-life anyway?

[mkoeppe]

It's not Debian-specific.

I got here on the route SciPy -> https://github.com/FFY00/meson-python -> patchelf, and SciPy purports to support GCC 6.3 https://docs.scipy.org/doc/scipy/dev/toolchain.html#c-language-standards @rgommers

[rgommers]

We actually just dropped that @mkoeppe. The link above is for the stable release, which does still support GCC 6.3 but also does not support C++17. In main we merged a PR that specifies we can start using C++17 but GCC 8 is the minimum: http://scipy.github.io/devdocs/dev/toolchain.html#compilers

[mkoeppe]

@rgommers, thanks for the info. Yes, it's affecting the build of stable scipy (1.9.x). I ran into it in https://trac.sagemath.org/ticket/34081

@Mic92 I don't need a solution here. We are just using patchelf 0.13.1 until we can bump our compiler requirements. I just wanted you to be aware that if you attempt to check C++17 support of the compiler, checking whether compiler accepts the -std=c++17 is not enough. You may want to use something like https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx_17.html