Update doctest

[btuin]

Hi,
SIGSTKSZ is no longer a statically defined variable, which makes doctest unable to compile.
This change was made by glibc 2.34. (https://sourceware.org/pipermail/libc-alpha/2021-March/123553.html)
doctest was updated to fix this issue: doctest/doctest#473
I successfully compiled the project using the latest version of doctest.

[cinemast]

Thank you for reporting this. I use alpine mostly now which does not use glibc. Which OS release did you currently use to reproduce this?

[btuin]

Sorry for the delay, I missed the notification.
I used Arch Linux, but it should be reproducible on any OS with glibc >= 2.34 (latest Ubuntu and Fedora for example).
Thank you for fixing this!