gengetopt: fix build with clang 16

pkgs/development/tools/misc/gengetopt/default.nix

···
       'set +o posix'
   '';
 
+  env = lib.optionalAttrs stdenv.cc.isClang {
+    CXXFLAGS = "-std=c++14";
+  };
+
   meta = {
     description = "Command-line option parser generator";