Merge pull request #310542 from rrbutani/fix/ucl-darwin

ucl: fix on Darwin/with `pkgsLLVM`

pkgs/development/libraries/ucl/default.nix

···
     sha256 = "b865299ffd45d73412293369c9754b07637680e5c826915f097577cd27350348";
   };
 
-  # needed to successfully compile with gcc 6
-  env.NIX_CFLAGS_COMPILE = lib.optionalString stdenv.cc.isGNU "-std=c90";
+  # needed to successfully compile with gcc 6+ and modern clang versions where
+  # `-Wimplicit-function-declaration` is otherwise on and errors by default
+  env.CFLAGS = "-std=c89";
 
   meta = {
     homepage = "http://www.oberhumer.com/opensource/ucl/";