ffmpeg: disable VDA on 10.10 unless clang is used

Fixes Homebrew/homebrew#33741.

Formula/ffmpeg.rb

@@ -72,7 +72,6 @@ class Ffmpeg < Formula
             "--enable-nonfree",
             "--enable-hardcoded-tables",
             "--enable-avresample",
-            "--enable-vda",
             "--cc=#{ENV.cc}",
             "--host-cflags=#{ENV.cflags}",
             "--host-ldflags=#{ENV.ldflags}"
@@ -110,6 +109,16 @@ class Ffmpeg < Formula
       args << "--extra-cflags=" + %x[pkg-config --cflags libopenjpeg].chomp
     end
 
+    # A bug in a dispatch header on 10.10, included via CoreFoundation,
+    # prevents GCC from building VDA support. GCC has no probles on
+    # 10.9 and earlier.
+    # See: https://github.com/Homebrew/homebrew/issues/33741
+    if MacOS.version < :yosemite || ENV.compiler == :clang
+      args << "--enable-vda"
+    else
+      args << "--disable-vda"
+    end
+
     # For 32-bit compilation under gcc 4.2, see:
     # http://trac.macports.org/ticket/20938#comment:22
     ENV.append_to_cflags "-mdynamic-no-pic" if Hardware.is_32_bit? && Hardware::CPU.intel? && ENV.compiler == :clang