Use libmagic.pc for compilation

[kelson42]

We use a somehow old way to detect magic library and it as been probably made this way as libmagic didn't provide a libmagic.pc at the time. Now it is the case so we would better fix the dependency to libmagic.

From #343

[kelson42]

We can not fully implement this ticket as long as Ubuntu 20.04 Focal has not move to libmagic/dev version 5.39+. Check https://launchpad.net/ubuntu/+source/file and https://bugs.launchpad.net/ubuntu/+source/file/+bug/1951594.

For the moment the code use the standard way except for Focal (and older systems). Once Focal has moved on to version 5.39, we will be able to remove the custom code in Meson configuration file.

[kelson42]

It seems we will have to wait EOS of Ubuntu 20.04 Focal n 04/2025 or move forward ourself earlier (stop supporting Ubuntu 20.04 Focal).