Guix manifest with vendor-neutral OpenGL dispatch

vngl.scm

;; This is a proof-of-concept and does not actually work (yet!).
;; The idea is that 'guix environment --ad-hoc -m vngl.scm -- glxgears' should
;; succeed even on unsupported graphics drivers, using the "foreign" libGL.

(use-modules (guix packages)
             (guix utils)
             (gnu packages gl))

(define vendor-neutral-mesa
  (package/inherit mesa
    (propagated-inputs
     `(("libglvnd" ,libglvnd)
       ,@(package-propagated-inputs mesa)))
    (arguments
     (substitute-keyword-arguments (package-arguments mesa)
       ((#:configure-flags flags)
        `(cons "-Dglvnd=true" ,flags))))))

(define with-vendor-neutral-mesa
  (package-input-rewriting `((,mesa . ,vendor-neutral-mesa))))

(packages->manifest
 (list (with-vendor-neutral-mesa mesa-utils)))