gecode_3: fix build with clang 16

pkgs/development/libraries/gecode/3.nix

···
     (import ./fix-const-weights-clang-patch.nix fetchpatch)
   ];
 
+  postPatch = ''
+    substituteInPlace gecode/flatzinc/lexer.yy.cpp \
+      --replace "register " ""
+  '';
+
   nativeBuildInputs = [ perl ];
 
   preConfigure = "patchShebangs configure";
+
+  env.CXXFLAGS = lib.optionalString stdenv.cc.isClang "-std=c++14";
 
   meta = with lib; {
     license = licenses.mit;