autogen.sh: Do not use --enable-maintainer-mode

* autogen.rc (maintainer_mode_option): New.
* autogen.sh: Act upon a value of off.  And use the variable instead
of --enable-maintainer-mode.

When building with docker we mount the source directory read-only and
thus we need to avoid any changes to the source directory to keep the
make running.

./autogen.sh must be run outside of the docker container.
./autogen.sh --build-w64 must be run inside the docker container