gnu: Use ffmpeg@3.4 in packages not ready for 4.0.

* gnu/packages/cdrom.scm (dvdstyler)[inputs]: Change FFMPEG with FFMPEG-3.4.
* gnu/packages/video.scm (mlt, motion, simplescreenrecorder)[inputs]: Likewise.
* gnu/packages/music.scm (moc, pianobar)[inputs]: Likewise.

gnu/packages/cdrom.scm

@@ -392,7 +392,7 @@ capacity is user-selectable.")
        ("fontconfig" ,fontconfig)
        ("libexif" ,libexif)
        ("libjpeg" ,libjpeg)
-       ("ffmpeg" ,ffmpeg)))
+       ("ffmpeg" ,ffmpeg-3.4)))
     (native-inputs
      `(("pkg-config" ,pkg-config)
        ("flex" ,flex)

gnu/packages/music.scm

@@ -2391,7 +2391,7 @@ tune-in sender list from @url{http://opml.radiotime.com}.")
        ("curl" ,curl)
        ("libgcrypt" ,libgcrypt)
        ("json-c" ,json-c)
-       ("ffmpeg" ,ffmpeg)))
+       ("ffmpeg" ,ffmpeg-3.4)))
     (native-inputs
      `(("pkg-config" ,pkg-config)))
     (home-page "http://6xq.net/projects/pianobar/")
@@ -2792,7 +2792,7 @@ available memory.")
      `(("alsa-lib" ,alsa-lib)
        ("curl" ,curl)
        ("faad2" ,faad2)
-       ("ffmpeg" ,ffmpeg)
+       ("ffmpeg" ,ffmpeg-3.4)
        ("file" ,file)
        ("jack" ,jack-1)
        ("libid3tag" ,libid3tag)

gnu/packages/video.scm

@@ -1847,7 +1847,7 @@ from sites like Twitch.tv and pipes them into a video player of choice.")
             #t)))))
     (inputs
      `(("alsa-lib" ,alsa-lib)
-       ("ffmpeg" ,ffmpeg)
+       ("ffmpeg" ,ffmpeg-3.4)
        ("fftw" ,fftw)
        ("libxml2" ,libxml2)
        ("jack" ,jack-1)
@@ -2082,7 +2082,7 @@ making @dfn{screencasts}.")
     ;; As a result, they are omitted. Please add them back if problems appear.
     (inputs
      `(("alsa-lib" ,alsa-lib)
-       ("ffmpeg" ,ffmpeg)
+       ("ffmpeg" ,ffmpeg-3.4)
        ("glu" ,glu)
        ("jack" ,jack-1)
        ("libxi" ,libxi)
@@ -2794,7 +2794,7 @@ It counts more than 100 plugins.")
        ("pkg-config" ,pkg-config)))
     (inputs
      `(("libjpeg" ,libjpeg)
-       ("ffmpeg" ,ffmpeg)
+       ("ffmpeg" ,ffmpeg-3.4)
        ("sqlite" ,sqlite)))
     (arguments
      '(#:phases (modify-phases %standard-phases