build: Use datarootdir in Meson-generated pkg-config files
With datadir outside of prefix (currently unsupported by Meson¹ but a frequent occurrence in Nixpkgs), the datadir entry, in the polkit-gobject-1 pkg-config file will be an absolute path. This will prevent changing the base directory of policydir with --define-variable=prefix=…, which many projects use to install policy files to their own prefix.
Previously, this worked without changes on Nixpkgs’s part because the pkg-config template used by Autotools contained @datarootdir@, which resolves to $(prefix)/share², taking no heed of the changed datadir.
Similar issue can happen when a distribution package redefines datadir like Debian does/did: https://blogs.gnome.org/hughsie/2014/06/16/datarootdir-v-s-datadir/
This patch changes Meson-based build system to use $(prefix)/share in the generated pkg-config files, mirroring Autotools.