I have made a change to how functions ("callbacks") are passed to GSL. Previously, it was necessary to write the function(s) as separate defun(s) and then define make-callback on them. Now, you simply pass a function designator (function object or symbol representing a function object). For example, to integrate sine from 0 to pi,
(integration-qng 'sin 0.0d0 pi)
or to integrate the sine of the square root of x,
(integration-qng (lambda (x) (sin (sqrt x))) 0.0d0 pi)
This goes for all functions and mobjects that pass functions to GSL. Many functions that took lists of functions (such as function, df, fdf) now take the functions separately. The argument 'scalarsp is present in many mobject creators; if set to T, your function(s) will be passed array arguments as separate elements, and should return multiple values of elements in proper order for any returned array(s).
If you are using any of these definitions, you will need to remove your make-callback forms and update the calling after you pull the new master branch.
Liam