Expose more directories in pc file

datadir can be overridden, in which case it is not possible to find the files using just prefix. This commit adds the datadir to the pkg-config file.

While at it, we also added jsondir, xmldir and localedir so that consumers do not have to construct the paths themselves.

Finally, we are relying the default localedir variable from Automake, in order to prevent discrepancy between the installed path and the pkg-config file. The issue occurred because localedir defaults to ${prefix}/share/locale in Automake and Autoconf (the latter produces the pc file) but we were overriding it to ${datadir}/locale just in Automake files.