opal: build with gcc6

build on gcc7 failed with
"Error: call of overloaded 'abs(unsigned int)' is ambiguous"

pkgs/top-level/all-packages.nix

@@ -10809,6 +10809,7 @@ with pkgs;
 
   opal = callPackage ../development/libraries/opal {
     ffmpeg = ffmpeg_2;
+    stdenv = overrideCC stdenv gcc6;
   };
 
   openh264 = callPackage ../development/libraries/openh264 { };