Okay, I just cloned json-glib, cd-ed into it, made a directory called ‘_build’, and attempted to run the commands you listed. I got as far as ’
ERROR: Target 'Gir' can't be generated as 'g-ir-scanner' could not be found
That seems to be something in the GObject-Introspection dependency, which the README.md says is optional.
So, to sum it up:
git clone https://gitlab.gnome.org/GNOME/json-glib
cd json-lib
mkdir _build
meson _build .
…and that’s as far as I get. Gotta go, datenight with my best friend…