mpd: enable ffmpeg support

mpd out of the box doesn't do well with m4a files. Since Homebrew
is used on OS X, and a lot of Mac users are bound to have lots of
music purchased from iTunes, it makes sense to install mpd with
all the dependencies necessary to play iTunes-purchased music.

Closes Homebrew/homebrew#15788.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/mpd.rb

@@ -13,6 +13,7 @@ class Mpd < Formula
   depends_on 'pkg-config' => :build
   depends_on 'glib'
   depends_on 'libid3tag'
+  depends_on 'ffmpeg'
   depends_on 'flac'
   depends_on 'libshout'
   depends_on 'mad'
@@ -31,6 +32,7 @@ class Mpd < Formula
     args = ["--disable-debug", "--disable-dependency-tracking",
             "--prefix=#{prefix}",
             "--enable-bzip2",
+            "--enable-ffmpeg",
             "--enable-flac",
             "--enable-shout",
             "--enable-fluidsynth",