Patch for the fact that all llvm python scripts are python 2.x compatible.

[罗勇刚(Yonggang Luo)]

Please take a view of this patch,
This patch solved a problem that when both Python 2.x and Python 3.x
installed on the Windows, the CMake detect the python 3.x executable.



Force to use python 2.x series of python.
Because under Win32, that's possible both Python 2.x and Python 3.x installed.
and then FindPythonInterp will found python 3.x by default.
So we use set(Python_ADDITIONAL_VERSIONS 2.7 2.6 2.5 2.4)
to force use 2.x series of Python if both Python 2.x and Python 3.x exist.


--
         此致
礼
罗勇刚
Yours
    sincerely,
Yonggang Luo