[PATCH] build: eliminate use of python execfile command

[David Bremner]

David Bremner <david at tethera.net> writes:

> As discussed in
> id:8cc9dd580ad672527e12f43706f9803b2c8e99d8.1405220724.git.wking at 
> tremily.us,
> execfile is unavailable in python3.

Pushed, with commit message reformatted.

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