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.
This is a missing part of LLDB commit r207122.
Patch by Brooks Davis in FreeBSD ports commit r353052
llvm-svn: 208204