Add conditioned-out diagnostics in columns_compute_width.

Add conditioned-out diagnostics in columns_compute_width.

These are a slightly cleaned-up version of the diagnostics I was using
to debug the layout problems in the GTK3 config box the other day. In
particular, if the box comes out far too wide - as I've just found out
that it also does when I compile the current state of the code against
OS X GTK3 - these diagnostics should provide enough information to
figure out which control is the limiting factor.

To enable: make CPPFLAGS="-DCOLUMNS_WIDTH_DIAGNOSTICS"