Tk_SafeInit() forward compatibility

In a header file of your C/C++ application or extension that includes the Tk library, add these lines:

  #include <tcl.h>
  #if (TK_MAJOR_VERSION < 8)
  #define Tk_SafeInit ((Tcl_PackageInitProc *) NULL)

Why? Now your code can safely include the initialization sequence:

  if (Tk_Init(interp) == TCL_ERROR) {
      return TCL_ERROR;
  Tcl_StaticPackage(interp, "Tk", Tk_Init, Tk_SafeInit);

without worrying about what versions of Tk do and do not define Tk_SafeInit().