#2700 tkPlatDecls.h no longer C++ friendly

[Don Porter]

For a few public Tk routines, such as Tk_GetHWND(),
it is documented that the header file tkPlatDecls.h
need to be #include-d.

Appears that a side-effect of some CONST-ifying
of the stubs machinery was to remove some
extern "C" wrapping. The consequence is that
#include <tkPlatDecls.h> in a C++ compile no
longer works.

Attached is a patch from kbk, to review and test.

Assigning to the source of the tkPlatDecls.h
revisions so we can check we're not undoing
anything important with the patch.

Thanks to Georgios Petasis for reporting
the problem.

[Don Porter]

[Jan Nijtmans]

Reviewed and tested: OK (FWIW)

[Don Porter]

fixed in HEAD