gts: fix Darwin build by setting -std=gnu17

The autotools bump has meant various packages are being built with 
-std=gnu23, often for the first time. I haven't nailed down the exact 
cause of the breakage, but it's not terribly surprising that code from 
2006 isn't coping, so just set it to gnu17 instead.

pkgs/by-name/gt/gts/package.nix

@@ -32,6 +32,11 @@ stdenv.mkDerivation (finalAttrs: {
   buildInputs = [ gettext ];
   propagatedBuildInputs = [ glib ];
 
+  env = {
+    # Doesn't build on Darwin with -std=gnu23. Apply uniformly as C standard target is something unlikely to vary across platforms.
+    NIX_CFLAGS_COMPILE = "-std=gnu17";
+  };
+
   doCheck = false; # fails with "permission denied"
 
   preBuild = lib.optionalString (stdenv.hostPlatform != stdenv.buildPlatform) ''