I am using an old version of gentoo linux. My distribution is not at all up to date. And this is wanted : I use old versions of xorg / mesa. The newers versions caused tearing issues on my 15kz crt TV (but I admit that I have not retested recent versions for a while to see if the problem still exists).
I will find a solution (maybe install a newer version of gcc in a new slot, or upgrade my distribution, and find another solution for this tearing problem).