Add a setup hook for fixing dylib install names on Darwin

Install names need to be absolute paths, otherwise programs that link
against the dylib won't work without setting $DYLD_LIBRARY_PATH.  Most
packages do this correctly, but some (like Boost and ICU) do not.
This setup hook absolutizes all install names.

pkgs/development/libraries/icu/default.nix

@@ -1,4 +1,4 @@
-{stdenv, fetchurl}:
+{ stdenv, fetchurl, fixDarwinDylibNames }:
 
 let
 
@@ -16,6 +16,10 @@ stdenv.mkDerivation {
     sha256 = "14l0kl17nirc34frcybzg0snknaks23abhdxkmsqg3k9sil5wk9g";
   };
 
+  # FIXME: This fixes dylib references in the dylibs themselves, but
+  # not in the programs in $out/bin.
+  buildInputs = stdenv.lib.optional stdenv.isDarwin fixDarwinDylibNames;
+
   postUnpack = ''
     sourceRoot=''${sourceRoot}/source
     echo Source root reset to ''${sourceRoot}