remind: fix darwin build

pkgs/tools/misc/remind/default.nix

@@ -1,4 +1,5 @@
 { lib
+, stdenv
 , fetchurl
 , tk
 , tcllib
@@ -28,6 +29,15 @@ tcl.mkTclDerivation rec {
       --replace-fail 'set Rem2PDF "rem2pdf"' "set Rem2PDF \"$out/bin/rem2pdf\""
   '';
 
+  env.NIX_CFLAGS_COMPILE = lib.optionalString stdenv.isDarwin (toString [
+    # Disable clang link time optimization until the following issue is resolved:
+    # https://github.com/NixOS/nixpkgs/issues/19098
+    "-fno-lto"
+    # On Darwin setenv and unsetenv are defined in stdlib.h from libSystem
+    "-DHAVE_SETENV"
+    "-DHAVE_UNSETENV"
+  ]);
+
   meta = with lib; {
     homepage = "https://dianne.skoll.ca/projects/remind/";
     description = "Sophisticated calendar and alarm program for the console";