Put extern "C" guards around all stub table struct definitions, 
so it is usable to be used for C++ compilers as well without the danger of modifying the calling convention. 
For tkDecls.h, ttkDecls.h tkIntXlibDecls.h is was no problem, because tk.h, tkTheme.h and Xlib.h
already contain those guards. But not for the other *Decls.h files.