compiling gtk 3.20 on debian jessie

[dagecko free fr]

Good afternoon,

I am facing an issue while wanting to compile Gtk 3.20 on my debian jessie machine.
I followed the instructions delivered on your website that can be found here: 
https://developer.gnome.org/gtk3/stable/gtk-building.html

Everything seems to work fine until I am building Gtk itself where I am facing a strange error about 
glib-codegen that cannot find the codegen module.
So I tried to check the options in glib configure but could not find anything relevant (I did the same for 
the Gtk configure script). I tried some approaches that I could have found when searching on the internet 
(changing python to version 2), but that did not help.

Do you have any clue on how I could achieve to complete the full building of Gtk ?

For information, I need Gtk 3.18 at least in order to have the OpenGL support. I am indeed migrating my UI 
editor that was made using Gtk 2.x and Gtkglext.
But since Glade on debian jessie does not support Gtk 2.x anymore, I have to deal with Gtk 3.18 or superior 
(in order to have the new GlArea). Unfortunately, on debian jessie only Gtk 3.14 is provided, so this widget 
is not available.

Also, I am not interested in moving to debian testing in order to keep a stable environment for development. 
I also would like to avoid compiling an old version of Glade since this implies to remain on Gtk 2.x which 
would be pity to my opinion.
There is an unofficial port of Gtkglext for Gtk 3.x but this is unofficial, and I am concretely using Gtkmm, 
and AFAIK there is no port of Gtkglextmm for Gtkmm 3.x. And even if such an unofficial port exists, this is a 
solution I'd like to avoid (the UI is ported on different OSes).
I could also move to Qt but I would like to avoid this solution as much as possible.

So my last solution was to build Gtk 3.18 (or 3.20 in my case) under my machine.

If that might be helpful, please note that I still have normal Gtk packages installed (obviously), and I use 
/usr/local as the installation directory for the new version.

Thanks a lot in advance.