libfpx: fix Darwin build

pkgs/development/libraries/libfpx/default.nix

@@ -8,6 +8,14 @@ stdenv.mkDerivation rec {
     sha256 = "0pbvxbp30zqjpc0q71qbl15cb47py74c4d6a8qv1mqa6j81pb233";
   };
 
+  # Darwin gets misdetected as Windows without this
+  NIX_CFLAGS_COMPILE = if stdenv.isDarwin then "-D__unix" else null;
+
+  # This dead code causes a duplicate symbol error in Clang so just remove it
+  postPatch = if (stdenv.cc.cc.isClang or false) then ''
+    substituteInPlace jpeg/ejpeg.h --replace "int No_JPEG_Header_Flag" ""
+  '' else null;
+
   meta = with stdenv.lib; {
     homepage = http://www.imagemagick.org;
     description = "A library for manipulating FlashPIX images";