diff --git a/pkgs/tools/misc/mcrypt/default.nix b/pkgs/tools/misc/mcrypt/default.nix
index ef5b8e56fa0a..c5013eb46a04 100644
--- a/pkgs/tools/misc/mcrypt/default.nix
+++ b/pkgs/tools/misc/mcrypt/default.nix
@@ -19,6 +19,10 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ libmcrypt libmhash ];
 
+  env = lib.optionalAttrs stdenv.cc.isClang {
+    NIX_CFLAGS_COMPILE = "-Wno-implicit-function-declaration";
+  };
+
   meta = {
     description = "Replacement for old UNIX crypt(1)";
     longDescription = ''