Kivy installation does not find GL/gl.h?

Install libgl1-mesa-dev package through apt. Also maybe you would wish to look on building instructions at launchpad: http://bazaar.launchpad.net/~kivy-team/kivy/packaging/view/head:/debian/control

You probably have to install mesa-common-dev, also see: Debian packages search.
If you have a Nvidia card, you may need to install nvidia-glx-dev.
You may experiment the same issue with the file glut.h, which is included in freeglut3-dev. Again, see the Debian packages which contain that file.