configure.ac: also print ${datarootdir} in summary

It goes something like:

    ${XLOCALEDIR} ->
    ${datarootdir}/locale/X11 ->
    ${prefix}/share/locale/X11 ->
    /usr/share/locale/X11

But AC_MSG_RESULT only performs one substitution. Instead of starting to
expand this stuff manually, just print datarootdir so all the pieces are
there.

Signed-off-by: Ran Benita <ran234@gmail.com>