Support C++20

[agagniere]

Fix compilation errors that arise when compiling with the c++20 standard.

To compile with --std=c++20, one can do:

cmake . -DCMAKE_CXX_STANDARD=20 -DCMAKE_CXX_EXTENSIONS=NO

We can check that the flag is used:

$ make -j `nproc` VERBOSE=1
... -fPIC -std=c++20 ...

Tested with gcc 9.5.0 and gcc 12.1.0 on ubuntu 22.04

[agagniere]

Tested on MacOS with Apple clang 14.
However for the compilation not to fail (unrelated to this PR), one needs to use the flag --std=gnu++20, id est:

cmake . -DCMAKE_CXX_STANDARD=20 # (implied:) -DCMAKE_CXX_EXTENSIONS=YES