New comment by ackalker on void-packages repository

https://github.com/void-linux/void-packages/issues/27965#issuecomment-766400380

Comment:
From the commits I mentioned before and some more digging in the [documentation for `configure_file()`](https://mesonbuild.com/Reference-manual.html#configure_file), I found:

* `install_dir` expects a subdirectory _relative_ to `prefix`.
* `configuration` substitution rule for `DATADIR` inside the JSON files needs _absolute_ path for the files to be usable.
* As `datadir` is not configured, it defaults to `share` which is a relative path.
* `qemu_datadir` is derived from the value of `datadir`, so it ends up a relative path as well
* `configuration` and `install_dir` end up using relative paths also

The way things are curently setup in Qemu's build system, setting `datadir` to an absolute path would also make `qemu_datadir` absolute, but I don't know if this violates the requirements for `configure_file()`.