GLM 0.9.4.1 fixes various bugs and clarifies quaternion interpolations. There are now three different functions now: mix, slerp and lerp.
lerp performs a linear interpolation between two quaternions. This operation is only defined with interpolation factors between [0, 1] and doesn't perform the rotation at constant speed. slerp perform a spherical linear interpolation between two quaternion. It always takes the short rotation path and it is performed at constant speed for interpolation factors between ]-inf, +inf[. This function is similar to the old shortMix function. mix is an oriented spherical linear interpolation between two quaternion. It is performed at constant speed for interpolation factors between ]-inf, +inf[.