Fixes for Clang 3.3 on Mac OS X 10.9

Mac OS 10.9 does now ship with Clang 3.3 which should be able to compile OGS (which is a good thing because compiling with gcc is broken at the moment, see #250). In order to compile OGS with clang I had to make these adjustments:

- Updated gtest to 1.7
- Fixed some platform checks in logog

Unfortunately some tests do not compile yet, see [this gist](https://gist.github.com/bilke/7233916) for the error output. Maybe this is something for @endJunction.