Make nx's CMake build file independent of NoLifeStory master project

We also remove the unconditional libc++ requirement, but this
involves rewriting the other CMake files, becuase every NoLifeStory
subproject requires libc++ when building with Clang. So we remove
the enable_cxx11 macro and put its commands into the top level
CMakeLists.txt, so C++11 is globally enabled when building NoLifeStory.

Since NoLifeNx also requires C++11, we also put C++11 enablement code
directly into NoLifeNx's CMakeLists.txt
nx: Properly identify GCC compiler, add proper install targets
nx: Don't install *_impl header files