ffmpeg@2.8: backport clock_gettime fix

Formula/ffmpeg@2.8.rb

@@ -72,6 +72,12 @@ class FfmpegAT28 < Formula
   depends_on "dcadec" => :optional
 
   def install
+    # Fixes "dyld: lazy symbol binding failed: Symbol not found: _clock_gettime"
+    if MacOS.version == "10.11" && MacOS::Xcode.installed? && MacOS::Xcode.version >= "8.0"
+      inreplace %w[libavdevice/v4l2.c libavutil/time.c], "HAVE_CLOCK_GETTIME",
+                                                         "UNDEFINED_GIBBERISH"
+    end
+
     args = ["--prefix=#{prefix}",
             "--enable-shared",
             "--enable-pthreads",