cjson: fix build on darwin

pkgs/development/libraries/cjson/default.nix

@@ -13,6 +13,12 @@ stdenv.mkDerivation rec {
 
   nativeBuildInputs = [ cmake ];
 
+  # cJSON actually uses C99 standard, not C89
+  # https://github.com/DaveGamble/cJSON/issues/275
+  postPatch = ''
+    substituteInPlace CMakeLists.txt --replace -std=c89 -std=c99
+  '';
+
   meta = with lib; {
     homepage = "https://github.com/DaveGamble/cJSON";
     description = "Ultralightweight JSON parser in ANSI C";