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.

This is a missing part of LLDB commit r207122.
Patch by Brooks Davis in FreeBSD ports commit r353052


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@208204 91177308-0d34-0410-b5e6-96231b3b80d8