Update to a new snapshot, now on the 7.0 line. [0]

Depend explicitly on python2.7. [1]

Install clang headers as they are now believed to work properly with
FreeBSD headers.

PR: 225315 [0], 225144 [1]
Submitted by: sunpoet [1]
Differential Revision: https://reviews.freebsd.org/D13984