autobuild: Adjust autobuild for PY2/PY3 get_python_lib behaviour

autobuild: Adjust autobuild for PY2/PY3 get_python_lib behaviour

The resuls of get_python_lib are different between python2 & python3
and this results in autobuild generating the wrong PYTHONPATH with
python3.

python2
=======
print ("%s" % get_python_lib(standard_lib=1, prefix='/my/prefix'))
/my/prefix/lib64/python2.7

python3
print ("%s" % get_python_lib(standard_lib=1, prefix='/my/prefix'))
/my/prefix/lib/python3.6

But with addition of plat_specific param the results are the same

python2
=======
print ("%s" % get_python_lib(plat_specific=1, standard_lib=0, prefix='/my/prefix'))
/my/prefix/lib64/python2.7/site-packages

python3
=======
print ("%s" % get_python_lib(plat_specific=1, standard_lib=0, prefix='/my/prefix'))
/my/prefix/lib64/python3.6/site-packages

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>