Make build detect and complain when compile env is outdated

[qmonnet]

$ just cargo build
  Downloaded rustc-hash v2.1.0
  Downloaded unicode-width v0.2.0
  Downloaded annotate-snippets v0.11.5
  Downloaded bindgen v0.71.1
  Downloaded 4 crates (557.3 KB) in 0.17s
   Compiling clang-sys v1.8.1
   Compiling anstyle v1.0.10
   Compiling bindgen v0.71.1
   Compiling unicode-width v0.2.0
   Compiling rustc-hash v2.1.0
   Compiling dpdk v0.1.0 (/home/qmo/dev/dataplane/dpdk)
   Compiling net v0.0.1 (/home/qmo/dev/dataplane/net)
   Compiling scratch v0.1.0 (/home/qmo/dev/dataplane/scratch)
   Compiling annotate-snippets v0.11.5
   Compiling dpdk-sys v0.0.1 (/home/qmo/dev/dataplane/dpdk-sys)
error[E0432]: unresolved import `rte_eth_tx_offload`
   --> dpdk/src/dev.rs:387:13
    |
387 |         use rte_eth_tx_offload::*;
    |             ^^^^^^^^^^^^^^^^^^ use of undeclared crate or module `rte_eth_tx_offload`

error[E0432]: unresolved import `rte_eth_tx_offload`
   --> dpdk/src/dev.rs:449:13
    |
449 |         use rte_eth_tx_offload::*;
    |             ^^^^^^^^^^^^^^^^^^ use of undeclared crate or module `rte_eth_tx_offload`

error[E0432]: unresolved import `rte_eth_tx_offload`
   --> dpdk/src/dev.rs:503:13
    |
503 |         use rte_eth_tx_offload::*;
    |             ^^^^^^^^^^^^^^^^^^ use of undeclared crate or module `rte_eth_tx_offload`

For more information about this error, try `rustc --explain E0432`.
error: could not compile `dpdk` (lib) due to 3 previous errors
warning: build failed, waiting for other jobs to finish...
error: Recipe `cargo` failed with exit code 101

... Woops, I just needed a just --yes refresh-compile-env. But for a second I thought there was a bug somewhere.

To improve user experience, we should be able to check and tell when the compile environment is outdated and needs a refresh, before trying to build the project.

We could also have a similar check that detects when the /nix link is missing on the system, and display a message more friendly than cc not found.

[daniel-noland]

This is a very solid point (which has also been on my mind for a while).

I have a few potential solutions in mind but they will take a day or two to cook up.

I think keeping the compile-env's packaged versions of clang and lld aligned with the user's rustc is going to be a trick (although I have some ideas on that as well). Likely we will need to add a toolchain file and keep that in sync with the compile-env

[qmonnet]

This is one option. Another one could be to check (before building) whether there's a newer version for the compile-env available online, and simply warn the user that their version is outdated. Then if we get a splat like the one above, it's easier to remember to check the compile-env version first?