Meson: align settables to existing AC options. Fix discrepancy.

With AC, datadir can be set independently from datarootdir. So provide option to set datarootdir explicitely. Furthermore, provide an option to set pkgconfigdir explictely too (the current AC provided configure does not allow to set it with --with-pkgconfdir; so this is an extension).

Fix too a discrepancy: datarootdir and other dir variables were set to some value, but these values were not used for pkgconfig file. Use the same value everywhere.