Full, up to date information on how to contribute to libvirt can be found on the libvirt website:
https://libvirt.org/contribute.html
To build the same document locally, from the top level directory of your git clone run:
$ meson build $ ninja -C build
You'll find the freshly-built document in docs/contribute.html.
If meson setup fails because of missing dependencies, you can set
up your system by calling
$ sudo dnf builddep libvirt
if you're on a RHEL-based distribution or
$ sudo apt-get build-dep libvirt
if you're on a Debian-based one.
Note that, for the RHEL-based case, if you're on a machine where you haven't done any C development before, you will probably also need to run
$ sudo dnf install gcc make ninja-build rpm-build
You might still be missing some dependencies if your distribution is shipping an old libvirt version, but that will get you much closer to where you need to be to build successfully from source.