autotools: remove confusing variables ${datadir} and DATADIR.

Those variables were confusing because autoconf usually defines DATADIR to
PREFIX/share and gajim uses both defs.datadir=PREFIX/share/gajim and
gajim.DATA_DIR=PREFIX/share/gajim/data.

Now, autoconf will use PKGDATADIR for setting the (renamed) defs.basedir
variable which will be used to set gajimpaths['DATA_DIR'] and
gajimpaths['ICONS_DIR'] in configpaths.py

Gajim will also looks for the ${GAJIM_BASEDIR} environment variable instead of
${datadir} which was really too generic (no namespace).