ffmpeg_{4,6,7}: build with apple-sdk_15

Upstream is pretty good about using availability checks, and there
are runtime‐checked features for macOS 10.13, 11, and 12 that
are only available if we build with a modern SDK. This impacts,
e.g. hardware‐accelerated video decoding in mpv.

FFmpeg should still continue to build and run on all our supported
macOS releases, with runtime functionality being no worse than before
on older versions.

pkgs/development/libraries/ffmpeg/generic.nix

@@ -330,6 +330,7 @@
 /*
  *  Darwin
  */
+, apple-sdk_15
 , xcode # unfree contains metalcc and metallib
 /*
  *  Cuda Packages
@@ -716,6 +717,7 @@ stdenv.mkDerivation (finalAttrs: {
   ++ optionals withCudaNVCC [ cuda_nvcc ];
 
   buildInputs = []
+  ++ optionals stdenv.hostPlatform.isDarwin [ apple-sdk_15 ]
   ++ optionals withAlsa [ alsa-lib ]
   ++ optionals withAmf [ amf-headers ]
   ++ optionals withAom [ libaom ]