gnu: Add ffmpeg@3.3 and use it on packages that fail with 3.4.

* gnu/packages/gstreamer.scm (gst-libav)[inputs]: Change ffmpeg to ffmpeg-3.3.
* gnu/packages/messaging.scm (qtox)[inputs]: Change ffmpeg to ffmpeg-3.3.
* gnu/packages/video.scm (ffmpeg-3.3): New variable.

Signed-off-by: Marius Bakke <mbakke@fastmail.com>

gnu/packages/gstreamer.scm

@@ -409,7 +409,7 @@ (define-public gst-libav
        ("python" ,python)))
     (inputs
      `(("gst-plugins-base" ,gst-plugins-base)
-       ("ffmpeg" ,ffmpeg)
+       ("ffmpeg" ,ffmpeg-3.3)
        ("orc" ,orc)
        ("zlib" ,zlib)))
     (home-page "http://gstreamer.freedesktop.org/")

gnu/packages/messaging.scm

@@ -822,7 +822,7 @@ (define-public qtox
                (("TIMESTAMP") "\"\""))
              #t)))))
     (inputs
-     `(("ffmpeg" ,ffmpeg)
+     `(("ffmpeg" ,ffmpeg-3.3)
        ("glib" ,glib)
        ("gtk+" ,gtk+-2)
        ("libsodium" ,libsodium)

gnu/packages/video.scm

@@ -751,6 +751,18 @@ (define-public ffmpeg
 audio/video codec library.")
     (license license:gpl2+)))
 
+(define-public ffmpeg-3.3
+  (package
+    (inherit ffmpeg)
+    (version "3.3.4")
+    (source (origin
+             (method url-fetch)
+             (uri (string-append "https://ffmpeg.org/releases/ffmpeg-"
+                                 version ".tar.xz"))
+             (sha256
+              (base32
+               "0mx9dvad3lkyhvsrblf280x2bz6dxajya1ylnspbdzldj0dpxfcq"))))))
+
 (define-public ffmpeg-2.8
   (package
     (inherit ffmpeg)