Even if build_ogl is set to =auto by default (only build if dependencies available) the absence of gtkglext doesn't turn that =auto to a =no resulting in a final complain about missing GtkGLExt even if nobody explicitly asked for OGL renderer.

Not sure how the =auto thing should behave here.
I mean, chances are that building sdl/ogl would work,
and building gtk/agg would work too.
So, should a renderer be disabled if it can't be built
for all GUIs or should a GUI be disabled in that case ?

Or should just a specific GUI/renderer combination be disabled ?