Generating gtk-doc on nicobar

[Mark Doliner]

We have a nightly cron script that runs as user 'mtnsupport' each
night and updates the API docs at https://developer.pidgin.im/doxygen/

In order to make the script work again for the gtkdoc-based
documentation in 3.0.0 we'll need to run autogen.sh and make. This
will require installing a bunch of dependencies on nicobar, and will
mean compiling Pidgin nightly. Does this seem reasonable to people?
Any objections?