Hello, Thanks for your answer but it didn't solve the problem.
Adding this section :
Section "Files"
ModulePath "/usr/lib/xorg/extra-modules"
ModulePath "/usr/lib/xorg/modules"
EndSection
to the xorg.conf doesn't change anything because
/usr/lib/i386-linux-gnu/xorg/extra-modules doesn't contain any file.
This command did solve the problem :apt-get install --reinstall xserver-xorg-video-intel libgl1-mesa-glx libgl1-mesa-dri
source = http://debianforum.de/forum/viewtopic.php?p=912902#p912902
