Meson: align settables to autotools ones. Fix discrepancy.

With autotools, datadir and datarootdir can be set independently. Add a meson option to set datarootdir (datadir is a built-in). Add to (this is an extension) a pkgconfigdir option to mimich the (unavailable) ---with-pkgconfigdir, but keep the current default as ${datadir}/pkgconfig.

There is a little acrobatics because the CI compares, byte by byte, the pc file produced by autotools and meson. So we let our default meson options with '${prefix}' and so on syntax, and replace the magic strings after the definition of the pc file since these are pkgconfig "magics".

Caveats: the added meson_options.txt has to be added to Makefile.am:EXTRA_DIST, so that it is correctly packaged (caught by CI).