Fixed the __APPLE__ check in platform.hpp

C/C++ development on OS X can happen with either GCC or Clang. GCC uses
the old standard library (libstdc++) by default, which puts the header
for unordered_map in std::tr1. Clang can use libstdc++ or the newer libc++,
which puts the header in std. These changes do the Right Thing for both.
Merge pull request #26 from eropple/master

Fix to allow compilation on OS X 10.5+