gnu: ffmpeg: Allow tests to run from the build tree.

* gnu/packages/video.scm (ffmpeg)[arguments]: Add 'set-ld-library-path'
  phase.

gnu/packages/video.scm

@@ -411,6 +411,16 @@ (define-public ffmpeg
                       "--disable-mipsdspr1"
                       "--disable-mipsdspr2"
                       "--disable-mipsfpu")))))
+         (add-before
+          'check 'set-ld-library-path
+          (lambda _
+            ;; Allow $(top_builddir)/ffmpeg to find its dependencies when
+            ;; running tests.
+            (let* ((dso  (find-files "." "\\.so$"))
+                   (path (string-join (map dirname dso) ":")))
+              (format #t "setting LD_LIBRARY_PATH to ~s~%" path)
+              (setenv "LD_LIBRARY_PATH" path)
+              #t)))
          (add-after
           'strip 'add-lib-to-runpath
           (lambda* (#:key outputs #:allow-other-keys)