A fresh pull request which includes the patch to compile S2E on a Mac and the tips 

Hi, I recreate the fresh pull request with three commits according to our last discussion:

1. A documentation patch about how to build S2E on a Mac.
2. A cocoa.m patch (duplicate definitions of "uint16") pulled from upstream QEMU. Because QEMU changed some .h files' location, I have to modify one single line of code pulled from upstream QEMU to make it compatible with your QEMU version.
3. A s2e_bdrv_fail patch. Maybe not the perfect patch, but it works. I explained the reason in the comment of my last commit.

I tested these patches on a Ubuntu system and it seems fine.