libtiff: fix build on darwin

Now that libtiff is using cmake, we need to let cmake
set the build rpath for the tests to pass on darwin.
The rpaths are rewritten at installation so
the output libraries should be unaffected.

pkgs/development/libraries/libtiff/default.nix

@@ -18,6 +18,10 @@ stdenv.mkDerivation rec {
     sha256 = "0d46bdvxdiv59lxnb0xz9ywm8arsr6xsapi5s6y6vnys2wjz6aax";
   };
 
+  cmakeFlags = if stdenv.isDarwin then [
+    "-DCMAKE_SKIP_BUILD_RPATH=OFF"
+  ] else null;
+
   # FreeImage needs this patch
   patches = [ ./headers.patch ];