Fix typo in #36
Fix typo in #36

Correctly interpolate PYTHON_EXECUTABLE string into python query
command.

Also saves PYTHON_SITE_PACKAGES in CMake's cache so that it can be
inspected later by `cmake -L`

mcopik/Elemental,birm/Elemental,justusc/Elemental,mcopik/Elemental,birm/Elemental,mcopik/Elemental,birm/Elemental,justusc/Elemental,mcopik/Elemental,mcopik/Elemental,justusc/Elemental,birm/Elemental,justusc/Elemental,justusc/Elemental,birm/Elemental,mcopik/Elemental,justusc/Elemental,birm/Elemental