Platforms still use the inefficient qvsnprintf() fall-back

qvsnprintf.cpp has a fall-back implementation in case QT_VSNPRINTF isn't defined by qplatformdefs.h. However, it's horribly slow: QString::asprintf().toLocal8Bit() + another memcy().

The qplatformdefs.h's of affected platforms contain

#if defined(_XOPEN_SOURCE) && (_XOPEN_SOURCE >= 500) 
#define QT_SNPRINTF             ::snprintf
#define QT_VSNPRINTF            ::vsnprintf
#endif

It seems we're either not setting the correct #define ahead of #include <unistd.h> or these #ifdef's are just outdated.

Since we kinda require C99 these days, the function should be universally available.

Acceptance criteria:

• The fall-back is removed and replaced by

  #ifndef QT_VSNPRINTF
  #error Need to define QT_VSNPRINTF in qplatformdefs.h.
  #endif