gnu: libva: Patch a dlopen call, set drivers path.

* gnu/packages/video.scm (libva): Patch the reference to libva-x11.so (passed
  to dlopen()) to use an absolute path.  Add --with-drivers-path= to
  #:configure-flags with a value pointing to mesa's $prefix/lib/dri.

gnu/packages/video.scm

@@ -264,6 +264,27 @@ SMPTE 314M.")
        ("libxext" ,libxext)
        ("libxfixes" ,libxfixes)
        ("mesa" ,mesa)))
+    (arguments
+     `(#:phases
+       (modify-phases %standard-phases
+         (add-before
+          'build 'fix-dlopen-paths
+          (lambda* (#:key outputs #:allow-other-keys)
+            (let ((out (assoc-ref outputs "out")))
+              (substitute* "va/drm/va_drm_auth_x11.c"
+                (("\"libva-x11\\.so\\.%d\"")
+                 (string-append "\"" out "/lib/libva-x11.so.%d\"")))))))
+       ;; Most drivers are in mesa's $prefix/lib/dri, so use that.  (Can be
+       ;; overridden at run-time via LIBVA_DRIVERS_PATH.)
+       #:configure-flags
+       (list (string-append "--with-drivers-path="
+                            (assoc-ref %build-inputs "mesa") "/lib/dri"))
+       ;; However, we can't write to mesa's store directory, so override the
+       ;; following make variable to install the dummy driver to libva's
+       ;; $prefix/lib/dri directory.
+       #:make-flags
+       (list (string-append "dummy_drv_video_ladir="
+                            (assoc-ref %outputs "out") "/lib/dri"))))
     (home-page "http://www.freedesktop.org/wiki/Software/vaapi/")
     (synopsis "Video acceleration library")
     (description "The main motivation for VA-API (Video Acceleration API) is