Debugging info unavailability

[Danny Milosavljevic]

Hi,

just now I had to debug a doxygen Segmentation Fault.  I tried to install 
doxygen:debug but that wasn't available.

I think it would be nice if these outputs were available by default (but not 
installed by default).

If we wanted to do that, we could just adapt guix/build-system/cmake.scm, 
guix/build-system/gnu.scm and guix/build-system/glib-or-gtk.scm outputs default 
to say '("out" "debug") instead of '("out").

Do we want to do that in one of the larger update cycles?