Handle -DBOUT_ENABLE_PYTHON=AUTO in the boutpp package.

[oparry-ukaea]

The BOUT++ BOUT_ENABLE_PYTHON def is currently handled with a boolean variant and used to set either -DBOUT_ENABLE_PYTHON=ON or -DBOUT_ENABLE_PYTHON=OFF.
Change it to a string variant and allow -DBOUT_ENABLE_PYTHON=AUTO.