[Bug middle-end/122913] [16 Regression?] pr12392.cpp -Ofast compile time went up 66% when switching to -std=c++20

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122913

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Ok, adding:
```
#include <bits/c++config.h>
#undef _GLIBCXX_EXTERN_TEMPLATE
#define _GLIBCXX_EXTERN_TEMPLATE 0
```
at the begining of the file caues -std=c++17 just become as slow as c++20.