Dependencies for "external LLVM"

[je-vv]

Same as aya-rs/bpf-linker#29, the book is missing to document the need for libpolly-15-dev, and apparently also the need for zlib1g-dev in the case of ubuntu 22.04.1. See Development Environment section.

[gwgrisk]

it works for me.

wget https://apt.llvm.org/llvm.sh && \
    chmod u+x llvm.sh && \
    sudo ./llvm.sh 15 && \
    sudo apt install libpolly-15-dev libz-dev

[bboyleonp666]

It seems now that's being added to the README in bpf-linker but not in the book, but that's for llvm 19. Perhaps we can update the book to follow the latest information.