[SCons] Fix an issue when using virtualenv
[SCons] Fix an issue when using virtualenv

Virtualenv substitutes an old version of site.py that may not have the
getusersitepackages function. However, it appears that USER_SITE is set
correctly in these cases.


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