The previous code concatenates includedir to _harfbuzz_prefix verbatim,
which results in a wrong final include path in case includedir is an absolute
path. Instead, we can let meson determine the absolute include and lib paths
in advance and save them in the cmake config.
This is an issue in nixpkgs, where includedir is set to the final (absolute)
path of the built library in the Nix store, which causes CMake projects
depending on harfbuzz to not configure.
See https://github.com/NixOS/nixpkgs/issues/180054.
Marco Rebhan
Ebrahim Byagowi
Volker Krause
Ben Boeckel