python: fixup python-config script

The original version uses sysconfig from the python interpreter
(normally host-python) which isn't really suited to provide information
of the target python.
Use the pure shell preprocessed backport from the upcoming python 3.4

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>