robodoc: fix darwin build

The makefile tried to be clever in Darwin environments, but by accident
included the same files multiple times in the `install` command. This is
not allowed these days.

pkgs/tools/text/robodoc/default.nix

@@ -15,12 +15,16 @@ stdenv.mkDerivation rec {
     sha256 = "l3prSdaGhOvXmZfCPbsZJNocO7y20zJjLQpajRTJOqE=";
   };
 
+  postConfigure = lib.optionalString stdenv.isDarwin ''
+    substituteInPlace Docs/makefile.am \
+      --replace 'man1_MANS = robodoc.1 robohdrs.1' 'man1_MANS ='
+  '';
+
   nativeBuildInputs = [ autoreconfHook ];
 
   hardeningDisable = [ "format" ];
 
   meta = with lib; {
-    broken = stdenv.isDarwin;
     homepage = "https://github.com/gumpu/ROBODoc";
     description = "Documentation Extraction Tool";
     longDescription = ''