Getting started guide && ./src/configure.sh

[randomizedcoder]

G'day,

I hope you are doing well

Very exciting to see the Ultra Ethernet protocols being added!

However, I suspect unintended "challenges" were introduced since the flake.nix was added.

Upon review, I found that actually xdp2 was no longer working on ubuntu without nix.

This pull request:

• Adds a getting started guide ( note - sample compiles challenges persist )
• Adds an experimental ./src/configure.sh designed as an alternative to the existing ./src/configure
• ./src/configure creates a config.mk that works on Ubuntu ( no nix ) using llvm or llvm20. ( The getting started guide explains that Ubuntu package naming making supporting different llvm versions tricky. configure.sh hopefully handles this correctly now. )
• ./src/configure.sh has extensive --debug_level 7 logging
• ./src/configure.sh modifies the test cpp code to support llvm 18 and 20
• Updates various Makefiles to allow the make variables to be passed, so -j$(nproc) works
• Enforces dependencies, so the builds work with -j$(nproc)

This has been tested with ubuntu24.0.4.3 ( no nix ) using llvm and llvm20, but I strongly encourage more testing

Happy to discuss

I'm going to keep working on the flake.nix, because it's broken, and this pull request marks the files as such

Thanks in advance,
Dave Seddon