[xml] Handling MSVC exports (was Release of libxml2-2.4.21)

["Vakoc, Mark" <Mark_Vakoc jdedwards com>]

> Sure. This would export all functions, even static ones, but 
> who cares?

I do.  /Zg reports the static keyword, so those functions can be filtered
out.

> What is more important a flaw, it would not export variables. In the
> default configuration, xmlFree is a pointer to a function and pointers
> are variables. 

Yes but all exported variables are already declared as LIBXML_DLL_IMPORT.  A
simple change to the definition of that to be __declspec(dllimport) when
building against libxml2, and __declspec(dllexport) when building libxml2
takes care of the variables.  No .def.src needed.