glm: Added package

OpenGL Mathematics (GLM) is a header only C++ mathematics library for
graphics software based on the OpenGL Shading Language (GLSL)
specification.

http://glm.g-truc.net/



Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>