We were assuming that meson project correspond to our projects and so we
were always building them, even if they were already compiled.
(This way, a simple `kiwix-build` is enough to recompile the WIP code of
our project).
However, on the CI, we do not archive the source code/build directory in
the base deps archive. So when we try to compile, the compile step of
meson projects fails because the source are not here.
We have a small workaround for pugixml but as zstd is also meson, it is
time to do something correct.
By default, all projects now try to skip if a build is already present.
Our projects are marked as `force_build` and so, they do not try to skip.
fb07b58812