ht: fix the build against gcc-15

Without the chnage the build fails on `master` as
https://hydra.nixos.org/build/324508162:

```
evalparse.c: In function 'yyparse':
evalparse.c:1340:16: error: too many arguments to function 'yylex'; expected 0, have 1
 1340 |       yychar = yylex (&yylval);
      |                ^~~~~  ~~~~~~~
In file included from eval.h:49,
                 from evalparse.y:9:
lex.h:10:5: note: declared here
   10 | int yylex();
      |     ^~~~~
```

pkgs/by-name/ht/ht/package.nix

@@ -24,6 +24,11 @@ stdenv.mkDerivation (finalAttrs: {
 
   env.NIX_CFLAGS_COMPILE = toString [ "-Wno-narrowing" ];
 
+  configureFlags = [
+    # Fails to build on -std=gnu23.
+    "CFLAGS=-std=gnu17"
+  ];
+
   meta = {
     description = "File editor/viewer/analyzer for executables";
     homepage = "https://hte.sourceforge.net";