udns: fix darwin build

pkgs/development/libraries/udns/default.nix

@@ -16,7 +16,20 @@ stdenv.mkDerivation rec {
     sha256 = "0447fv1hmb44nnchdn6p5pd9b44x8p5jn0ahw6crwbqsg7f0hl8i";
   };
 
+  # udns uses a very custom build and hardcodes a .so name in a few places.
+  # Instead of fighting with it to apply the standard dylib script, change
+  # the right place in the Makefile itself.
+  postPatch =
+    if stdenv.isDarwin
+    then
+      ''
+        substituteInPlace Makefile.in \
+          --replace --soname, -install_name,$out/lib/
+      ''
+    else "";
+
   installPhase = ''
+    runHook preInstall
     mkdir -p $out/bin
     mkdir -p $out/include
     mkdir -p $out/lib
@@ -30,6 +43,7 @@ stdenv.mkDerivation rec {
     ln -rs $out/lib/libudns.so.0 $out/lib/libudns.so
     cp dnsget.1 rblcheck.1 $out/share/man/man1
     cp udns.3 $out/share/man/man3
+    runHook postInstall
   '';
 
   # keep man3
@@ -40,7 +54,7 @@ stdenv.mkDerivation rec {
     description = "Async-capable DNS stub resolver library";
     license = licenses.lgpl21Plus;
     maintainers = [ maintainers.womfoo ];
-    platforms = platforms.linux;
+    platforms = platforms.unix;
   };
 
 }