Add rpath linker option to vibec

[chandralegend]

Summary

• ensure vibec built shared libraries know how to find libvibelang
• detect installation prefix from $PREFIX with fallback to /usr/local
• on macOS use @loader_path for rpath

Testing

• make test (fails: Bison/Flex missing)

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https://chatgpt.com/codex/tasks/task_e_6842cc257ac483328fbe4b7bdd9262f9

[chandralegend]

This is still not working, we still have to parse the gcc -o static_joke_example static_joke_example.c ./joke.so -lvibelang -Wl,-rpath,/usr/local/lib