lhapdf: fix compilation with gccStdenv on darwin

This is used to compile against MCFM that is built with GCC

pkgs/development/libraries/physics/lhapdf/default.nix

···
     sha256 = "sha256-fS8CZ+LWWw3e4EhVOzQtfIk6bbq+HjJsrWLeABDdgQw=";
   };
 
+  # The Apple SDK only exports locale_t from xlocale.h whereas glibc
+  # had decided that xlocale.h should be a part of locale.h
+  postPatch = lib.optionalString (stdenv.isDarwin && stdenv.cc.isGNU) ''
+    substituteInPlace src/GridPDF.cc --replace '#include <locale>' '#include <xlocale.h>'
+  '';
+
   nativeBuildInputs = [ makeWrapper ];
   buildInputs = [ python ];