patch re2 to fix build issue with libc++

Without these changes, building fteproxy on OS X 10.9 fails with "fatal error: 'tr1/unordered_set' file not found". Including and applying the patch from https://code.google.com/p/re2/issues/detail?id=76 fixes the issue.

https://ceres-solver-review.googlesource.com/#/c/4170/ (which turned up in a search for that error) explains:
> Apple switched to using libc++ with 10.9.0 which places unordered_map in std, breaking our assumptions about the platform.

In case it's worth following, there's some recent activity in the re2 project around fixing Darwin build issues. A trail can be followed from https://code.google.com/p/re2/issues/detail?id=99, which is how I found https://code.google.com/p/re2/issues/detail?id=76.

P.S. I'm the person from Brave New Software working on Lantern who introduced himself to you at Real World Crypto. Excited I'm getting to start looking at fteproxy now, and to be submitting a PR on my first night. :)