Keep symlinks when copying into .app bundle

I need this change for the current OSX code-signing process, which looks like:

1. Deterministically gitian-build an .osx.tar.gz
2. Extract the .dmg and then the .app from the .dmg
3. Re-run macdeployqtplus -sign using the extracted, deterministic .app to create a code-signed app inside a .dmg that is actually released.

Step 3 broke with OSX 10.9.5, because symbolic links inside the .app were not preserved. This fixes that.

@laanwj : this should be cherry-picked to the 0.9.3 branch in case there is a 0.9.4.

The plan is to change the code-signing process to something like:

1. Code-signer gitian builds first, producing a .osx.tar.gz that has a .dmg that is signed.
2. Everybody else uses the signatures produced in (1) as an input to the gitian-building process and verifies that end results match.