freedroid: set -std=gnu17 to fix gcc-15 build

Without the change the build fails on `master` as
https://hydra.nixos.org/build/319903242:

    struct.h:40:14: error: 'bool' cannot be defined via 'typedef'
       40 | typedef char bool;
          |              ^~~~

pkgs/by-name/fr/freedroid/package.nix

@@ -47,6 +47,9 @@ stdenv.mkDerivation rec {
     touch NEWS
   '';
 
+  # DOes not build on -std=c23 due to `bool` collision.
+  env.NIX_CFLAGS_COMPILE = "-std=gnu17";
+
   postInstall = ''
     mkdir -p $out/share/icons/hicolor/32x32/apps
     convert graphics/paraicon.bmp $out/share/icons/hicolor/32x32/apps/freedroid.png