mrtg: gcc14 broke detection of print format specifiers

pkgs/by-name/mr/mrtg/configure-long-long-format-gcc14.patch

@@ -0,0 +1,10 @@
+--- mrtg-2.17.10/configure.bak	2025-10-17 20:44:24.878718603 +0100
++++ mrtg-2.17-10/configure	2025-10-17 20:44:40.343747835 +0100
+@@ -3664,6 +3664,7 @@
+   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+ /* end confdefs.h.  */
+ #include <stdio.h>
++#include <stdlib.h>
+             int main()
+             {
+                 long long b, a = -0x3AFAFAFAFAFAFAFALL;

pkgs/by-name/mr/mrtg/package.nix

@@ -1,6 +1,7 @@
 {
   lib,
   stdenv,
+  makeWrapper,
   fetchurl,
   perl,
   gd,
@@ -24,6 +25,8 @@ stdenv.mkDerivation rec {
     sha256 = "sha256-x/EcteIXpQDYfuO10mxYqGUu28DTKRaIu3krAQ+uQ6w=";
   };
 
+  nativeBuildInputs = [ makeWrapper ];
+
   buildInputs = [
     # add support for ipv6 snmp:
     # https://github.com/oetiker/mrtg/blob/433ebfa5fc043971b46a5cd975fb642c76e3e49d/src/bin/mrtg#L331-L341
@@ -32,6 +35,20 @@ stdenv.mkDerivation rec {
     rrdtool
   ];
 
+  patches = [
+    # gcc14 broke detection of printf format specifiers
+    # building from master seems to be fixed upstream, so next release can (likely) drop the patch
+    # just keep the CFLAGS below
+    ./configure-long-long-format-gcc14.patch
+  ];
+  env.NIX_CFLAGS_COMPILE = "-Werror";
+  env.NIX_CFLAGS_LINK = "-lm";
+
+  postInstall = ''
+    # mrtg wants plain C locale
+    wrapProgram $out/bin/mrtg  --set LANG C
+  '';
+
   meta = with lib; {
     description = "Multi Router Traffic Grapher";
     homepage = "https://oss.oetiker.ch/mrtg/";