include/guiddef.h: C++ compatibility patch (with MSVC++ support)

[Troy Rollo]

The following patch corrects the definition for DEFINE_GUID when INITGUID is 
on and the current language is C++. In such a case an initialised variable 
declared as "const" is implicitly static unless it is explicitly declared as 
also "extern". This patch declares initialised GUIDs as "extern const" so 
that they are visible to other compilation units.

This patch differs from the earlier one in that not only does it support GNU C 
and C++, but also MSVC and MSVC++ (the latter of which was also not working 
previously due to MSVC++ mangling globals).

ChangeLog:
	For C++, declare initialised GUIDs to be "extern const" rather than just
	"const" to ensure that the initialised GUID is visible to other modules, and
       use EXTERN_C rather than just "extern" for the benefit of MSVC++, which
       does name mangling on global variables.
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