ucl: fix on Darwin/with pkgsLLVM

Modern clang versions error with `-Wimplicit-function-declaration`
[by default]; this causes `ucl`'s `configure` scripts check for ANSI C
headers (`stddef.h`) to fail because its test assumes `exit` is
implicitly declared leading to build failures:
```console
checking for ANSI C header files... no
```

[by default]: https://reviews.llvm.org/D122983

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/";