Jean-Michaël Celerier f09b24970d
Fix gsl/util for c++20 compilers without <span> (#993)
For instance, clang 10 sets __cplusplus >= 202002L yet does not have span, which causes build errors:

https://gcc.godbolt.org/z/Yq345zGea
2021-09-15 15:12:11 -07:00
..
gsl
2020-12-09 15:18:07 -08:00