Instead clone the libbpf repo at a specific hash during setup.
This is to fix an issue whereby submodules are not included
in the tarball and therefore won't be updated/fetched during
setup after unzipping the tarball.
Otherwise, we end up passing CC=ccache to libbpf's Makefile which triggers
an error as ccache invoked on its own can't act as a stand-in for the
compiler.
This is to potentinally reduce issues with folks
using different versions of libbpf at runtime.
This also:
- makes static linking of libbpf the default
- adds steps in `meson setup` to fetch libbpf and make it
