Proper OSX/CLANG/CXX11 support in stl_delete.h

- replaces fix in 066df0d218ee4391eb3e15259deca5ccc190ed84
- Check if _LIBCPP_VECTOR is defined for vector_delete.
  This is defined in libc++'s headers which Apple is using
  instead of libstdc++

I checked gcc-4.2, clang, and clang with c++11 enabled on Mac.  This one shouldn't break old gcc versions