Build nix-util with Meson

The idea is two-fold:

- Replace autotools with Meson

- Build each library in its own derivation

The interaction of these two features is that Meson's "subprojects"
feature (https://mesonbuild.com/Subprojects) allows us to have single
dev shell for building all libraries still, while also building things
separately. This allows us to break up the build without a huge
productivity lost.

I tested the Linux native build, and NetBSD and Windows cross builds.

Also do some clean ups of the Flake in the process of supporting new
jobs.

Special thanks to everyone that has worked on a Meson port so far,
@p01arst0rm and @Qyriad in particular.

Co-Authored-By: p01arst0rm <polar@ever3st.com>
Co-Authored-By: Artemis Tosini <lix@artem.ist>
Co-Authored-By: Artemis Tosini <me@artem.ist>
Co-Authored-By: Felix Uhl <felix.uhl@outlook.com>
Co-Authored-By: Jade Lovelace <lix@jade.fyi>
Co-Authored-By: Lunaphied <lunaphied@lunaphied.me>
Co-Authored-By: Maximilian Bosch <maximilian@mbosch.me>
Co-Authored-By: Pierre Bourdon <delroth@gmail.com>
Co-Authored-By: Qyriad <qyriad@qyriad.me>
Co-Authored-By: Rebecca Turner <rbt@sent.as>
Co-Authored-By: Winter <winter@winter.cafe>
Co-Authored-By: eldritch horrors <pennae@lix.systems>
Co-Authored-By: jade <lix@jade.fyi>
Co-Authored-By: julia <midnight@trainwit.ch>
Co-Authored-By: rebecca “wiggles” turner <rbt@sent.as>
Co-Authored-By: wiggles dog <rbt@sent.as>
Co-Authored-By: fricklerhandwerk <valentin@fricklerhandwerk.de>
Co-authored-By: Eli Schwartz <eschwartz93@gmail.com>
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

meson.build

@@ -0,0 +1,9 @@
+# This is just a stub project to include all the others as subprojects
+# for development shell purposes
+
+project('nix-dev-shell', 'cpp',
+  version : files('.version'),
+  subproject_dir : 'src',
+)
+
+subproject('libutil')