ffmpeg: fix build on 10.11

Fixes #4902.

Closes #4924.

Signed-off-by: ilovezfs <ilovezfs@icloud.com>

Formula/ffmpeg.rb

@@ -91,6 +91,12 @@ class Ffmpeg < Formula
   end
 
   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 = %W[
       --prefix=#{prefix}
       --enable-shared