[xml] win32 threads feedback

[Stéphane Bidoul <stephane bidoul softwareag com>]

Hi Igor,

I found out that the thread-enabled libxml2 does not work 
with the python bindings: it crashes on "if (!tlstate_inited) {".

I think it's because Compiler TLS does not work when the 
DLL is loaded through LoadLibrary (MSVC documentation).

Everything looks fine if libxml2 is compiled with threads=native,
which uses TlsAlloc & co.

So you may want to default to threads=native instead of 
threads=ctls, since it allows for a wider range of usage scenarios.

-sbi