Log Message:
math/volk: Work around boutique cmake python-finding

volk has an included cmake fragment to find python, and while it is
supposed to find recent 3.x, that apparently gets confused by the
presence of 2.7.  After noting that lang/python/pyversion.mk contains
N ways to force the python path for various cmake idioms, add a
forcing define in math/volk's Makefile following its boutique
python-finding.

Resolves build when python2.7 is present, and likely makes it more
robust against other situations with multiple symptoms.