The previous way of checking that _putenv_s is defined does not work,
because _putenv_s is a function and not a define. This is a partial
application of commit 31115e0369747b1e1c45cbe3f2a90f6dff66666a from
alexrj/Slic3r.
I tried cherry picking the whole commit, but unicode is already handled
diffrently here, so that would have been a lot of work.
