Unity

* Use the `__cplusplus`define to disable C++11 features if they are not available.
* New `UnityRelease` target in the XCode project, using `gnu++98` and `libstdc++` instead of `gnu++0x` and `libc++`.
* Use `libcurl.a` in addition of `libcurl_arm64.a` to make the sdk available for `armv7` and `armv7s`.
* Update the Makefile.
Does this imply the Unity implementation is not thread safe?