FFmpeg: Add option to build ffplay

The ffplay binary is only built by default if SDL is present---which made
obtaining it a hit-and-miss operation for some users. A new option,
`--with-ffplay`, has been added that will cause ffplay to be built.

ffplay will be explicitly disabled if the option is not invoked.

Fixes Homebrew/homebrew#6880.

Formula/ffmpeg.rb

@@ -1,5 +1,9 @@
 require 'formula'
 
+def ffplay?
+  ARGV.include? '--with-ffplay'
+end
+
 class Ffmpeg < Formula
   url 'http://ffmpeg.org/releases/ffmpeg-0.8.6.tar.bz2'
   homepage 'http://ffmpeg.org/'
@@ -9,6 +13,13 @@ class Ffmpeg < Formula
 
   fails_with_llvm 'Dies during compilation of motionpixels_tablegen'
 
+  def options
+    [
+      ["--with-tools", "Install additional FFmpeg tools."],
+      ["--with-ffplay", "Build ffplay."]
+    ]
+  end
+
   depends_on 'yasm' => :build
   depends_on 'x264' => :optional
   depends_on 'faac' => :optional
@@ -19,11 +30,7 @@ class Ffmpeg < Formula
   depends_on 'libvpx' => :optional
   depends_on 'xvid' => :optional
 
-  def options
-    [
-      ["--with-tools", "Install additional FFmpeg tools."]
-    ]
-  end
+  depends_on 'sdl' if ffplay?
 
   def install
     args = ["--prefix=#{prefix}",
@@ -41,6 +48,7 @@ class Ffmpeg < Formula
     args << "--enable-libvorbis" if Formula.factory('libvorbis').installed?
     args << "--enable-libvpx" if Formula.factory('libvpx').installed?
     args << "--enable-libxvid" if Formula.factory('xvid').installed?
+    args << "--disable-ffplay" unless ffplay?
 
     # For 32-bit compilation under gcc 4.2, see:
     # http://trac.macports.org/ticket/20938#comment:22