texinfo413: fix build on darwin

pkgs/development/tools/misc/texinfo/4.13a.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, texinfo, ncurses, xz }:
+{ stdenv, fetchurl, texinfo, ncurses, xz, lib }:
 
 stdenv.mkDerivation rec {
   pname = "texinfo";
@@ -12,6 +12,10 @@ stdenv.mkDerivation rec {
   buildInputs = [ ncurses ];
   nativeBuildInputs = [ xz ];
 
+  env.NIX_CFLAGS_COMPILE = toString (lib.optionals stdenv.cc.isClang [
+    "-Wno-error=implicit-function-declaration"
+  ]);
+
   # Disabled because we don't have zdiff in the stdenv bootstrap.
   #doCheck = true;