If CMake finds a python interpreter, use it

The FreeBSD package building cluster installs e.g. 'python2.7', but no
plain 'python' to avoid version-related issues.

CMake's FindPythonInterp locates an interpreter with such a name and
provides it in the PYTHON_EXECUTABLE variable.  Use that if it's set,
falling back to the original '/usr/bin/env python' otherwise.

Patch by Brooks Davis in FreeBSD ports commit r352012