[patch] Update the cmake python version check

[Rafael Espíndola]

The attached patch updates the handling of python in the cmake build:

* We now support python 3, so we don't need to special cases systems
with non "python" but with "python2" and "python3" interpreters.
* We were not checking the python version. With this patch we have the
same check as configure.

Cheers,
Rafael
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