build: eliminate use of python execfile command

build: eliminate use of python execfile command

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

The approach of this commit avoids modifying the python module path,
which is arguably preferable since it avoids potentially accidentally
importing a module from the wrong place.