[build] Check for python2.7 specifically

Not everyone has a python2 symlink, besides only 2.7 is being tested
against anyway.
[backend] We don’t need hb-ft.h here
[build] Make lxml module optional

It is much faster than xml.etree.ElementTree, but we can do without, so
use it only if it is present.
[build] Port token list generation to Python