python, python3: build with tzset support

This patch adds support for time.tzset() in Python 2.7 and Python 3.

The Python configure script sets ac_cv_working_tzset to 'no' when
cross compiling, although it should be available and work with all
toolchains supported by Buildroot (?).
The patch below pre-sets the variable so that time.tzset() works.

[Thomas: adjust commit log.]

Signed-off-by: Henry Margies <henry.margies@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>