Depending on python2.x for building

[Jeremy Huddleston]

So it turns out that llvm doesn't build well with python3 (https://trac.macports.org/ticket/32575).  It's been building fine in most cases without the python dependency in the port by picking up /usr/bin/python or ${prefix}/bin/python if a 2.x python is port selected.

What's the "best practices" way to handle this?  I don't want to use /usr/bin/python, and I don't want to deal with multiple python variants.  Is there a way to set $PYHON to "any" 2.x python on the system (and build one if there is none available)?