mcrypt: fix build on darwin

pkgs/tools/misc/mcrypt/default.nix

···
 
   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 = ''