Bug 3605401: Compiler error with latest mingw-w64 headers
[Bug 3605401]: Compiler error with latest mingw-w64 headers.
[Bug 3605401]: Compiler error with latest mingw-w64 headers.
Merge trunk. 
Convert Tcl_SetVar/Tcl_UnsetVar/Tcl_GetVar/Tcl_TraceVar to macros, calling the *2 variants of the function. No change of functionality.