Small fixes to the core C++ FlatBuffers implementation.

- Ensured weak linkage with the version string is not used on Windows,
  especially cygwin (which throws a linker error).
- Avoided a VS debug error for taking the address of the first element
  of an empty vector.
- Made copy/assignment constructors for downward_vector and
  FlatBufferBuilder private, to avoid people unintentionally making
  expensive copies.
- Using the more correct _WIN32 instead of WIN32

Change-Id: I801b5c8b159e3721af6d1ef0978a3247ba168bab
Tested: on Windows (VS + Cygwin) and Linux.