Hello Guixers!

Given the recent discussion about dropping python 2 completely I thought 
it's time to build our libboost against python 3. I didn't have the time 
to check if every boost dependency (there are a lot ;) ) supported 
python 3, so I added a new variable "boost-with-python3". I'm not sure 
if I should have made it hidden or if I should have update directly 
boost definition so any feedback is welcome.

Thanks for your patience reviewing this patch,

Giacomo