Fixing Python autodetection and adding a sanity check.
Fixing Python autodetection and adding a sanity check.

The script fails when Boost.Python3 is detected but Python 3 is not really
available.
The error is a regression from e93d8438.
PYTHON_EXECUTABLE mustn't be used because the PythonInterp script sets the
variable to any interpreter found even it has failed because the version
doesn't match the requested one.

shurikasa/CMake,ptoharia/CMake,ptoharia/CMake,ptoharia/CMake,biddisco/CMake,ptoharia/CMake,shurikasa/CMake,shurikasa/CMake,shurikasa/CMake,biddisco/CMake,biddisco/CMake,biddisco/CMake