Fixes OSX Build issues.

CMakeLists.txt changes:

 * Moved compiler configuration above gtest include
 * Forced libc++ when building with clang

src/EntityManager.hpp changes:

 * Switched stdint include string -- clang likes this one better.

src/World.hpp

 * Members requiring a template argument need the template keyword
   after the . or -> and before the member symbol.

All tests now pass on OSX with the following build process:

    mkdir build && cd build && cmake .. && cmake --build . && ./testrunner