build: add SAMBA_DATADIR as "samba" subdirectory of DATADIR

build: add SAMBA_DATADIR as "samba" subdirectory of DATADIR

DATADIR should have been set to this path from the beginning, too late to change
that now as ut's used as parent for two other directory varialbles: SETUPDIR and
CODEPAGEDIR.

From <https://www.gnu.org/prep/standards/html_node/Directory-Variables.html>:

  datadir

    The directory for installing idiosyncratic read-only
    architecture-independent data files for this program. This is usually the
    same place as ‘datarootdir’, but we use the two separate variables so that
    you can move these program-specific files without altering the location for
    Info files, man pages, etc.

    This should normally be /usr/local/share, but write it as
    $(datarootdir). (If you are using Autoconf, write it as ‘@datadir@’.)

    The definition of ‘datadir’ is the same for all packages, so you should
    install your data in a subdirectory thereof. Most packages install their
    data under $(datadir)/package-name/.

Currently Samba doesn't install any application specific data files, but I'm
going to do just that in a subsequent commit.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Noel Power <noel.power@suse.com>