diff --git a/pkgs/development/libraries/ucl/default.nix b/pkgs/development/libraries/ucl/default.nix
index 048f35868cda..d8e05e9374bf 100644
--- a/pkgs/development/libraries/ucl/default.nix
+++ b/pkgs/development/libraries/ucl/default.nix
@@ -9,8 +9,9 @@ stdenv.mkDerivation rec {
     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/";