Implemented John's suggestion.
Trac #16136: Prevent user from doing stupid thing with the doc building system

Right now there are at least two stupid thing the user can do which can
get him in trouble:

- type make in the `src/doc` directory : this **delete** the doc !
- type `sage docbuild reference/non_existing_doc` : this will create a
directory is the doc system that prevents the doc from compiling and
which even `make doc-clean` doesn't remove.

I'm adding two small safe guard to prevent those.

URL: http://trac.sagemath.org/16136
Reported by: hivert
Ticket author(s): Florent Hivert
Reviewer(s): Volker Braun