Update recipe for Conan 2.0 compat. Update packages.

[tarc]

This PR updates the Conan recipe and CMake script to conform to Conan 2 — cf. Conan migration guide to 2.0.

It also fixes a build issue when compiling without GNU extensions.

I'm sorry this PR seems a bit random and without purpose. I was playing with concore a while ago and then changed jobs and I completely forgot about it. As I was browsing my repos the other day I noticed those changes here may be handy for someone willing to package it or tweak dependency versions, build systems, etc.