[cmake] Add a DUNE_PYTHON_FORCE_PYTHON_VERSION variable

So far, a user could only distinguish python2 and 3. This makes it possible to also force a specific version given as a string.

Of course, the better solution would be if FindPythonInterp would accept a path hint just like every other find module does, but for some reason upstream is resistant to implementing this and I really don't want to ship our own find module for the python interpreter.

This fixes #118 (closed).

mentioned in issue #118 (closed)

Works, thank you!

Any updates on this? Is there a problem I can help with?

Non-invasive, simple to implement feature solving a valid user problem? Merging!

mentioned in commit 2df1f68c

merged

Thank you!