ffmpeg-full: fix the build phase

I was surprised the build is so slow, and I see it's single-threaded
due to all work being done in install phase :-/
Please, do *not* rewrite phases unless you're really confident it's
a good approach; typically postFoo and preFoo is much better.

I considered filing this ffmpeg-related string of changes as a PR,
but in the end it doesn't seem likely to need review and we have too
many PRs as it is...

(cherry picked from commit 5236232eea)

pkgs/development/libraries/ffmpeg-full/default.nix

@@ -420,8 +420,8 @@ stdenv.mkDerivation rec {
                                    MediaToolbox VideoDecodeAcceleration
                                    libiconv cf-private /* For _OBJC_EHTYPE_$_NSException */ ];
 
-  # Build qt-faststart executable
-  buildPhase = optional qtFaststartProgram ''make tools/qt-faststart'';
+  buildFlags = [ "all" ]
+    ++ optional qtFaststartProgram "tools/qt-faststart"; # Build qt-faststart executable
 
   # Hacky framework patching technique borrowed from the phantomjs2 package
   postInstall = optionalString qtFaststartProgram ''