CMake's built-in FindLua resolves to the versioned library file
(e.g. liblua-5.4.so) instead of the unversioned symlink (liblua.so),
causing a library name mismatch between the two build systems.
Add a custom cmake/FindLua.cmake that uses pkg-config — matching
configure.py's approach — and find_library(NAMES lua) to find the
unversioned symlink. This also mirrors the pattern used by other
Find modules in cmake/ (FindxxHash, Findlz4, etc.).
