[SCons] Add PYTHONPATH to list of default environment variables
[SCons] Add PYTHONPATH to list of default environment variables

Simplifies compilation when Python packages needed for compilation (e.g. Cython)
are not installed in system-wide locations.


imitrichev/cantera,Heathckliff/cantera,imitrichev/cantera,Heathckliff/cantera,Heathckliff/cantera,Heathckliff/cantera,imitrichev/cantera,Heathckliff/cantera,imitrichev/cantera,Heathckliff/cantera,imitrichev/cantera,imitrichev/cantera