[xml] windows static for dll builds

[Rob Richards <rrichards ctindustries net>]

I need to start using static builds with the LIBXML_STATIC_FOR_DLL flag 
and would like to make a couple of changes in order to facilitate 
building them.

First - change the calling convention of xmlDllMain. I have no idea how 
(or if) anyone is actually building with this flag, but xmlDllMain is 
using the stdcall calling convention, while everything else is built 
using the one specified by XMLCALL making it impossible to link to this 
function correctly.
BOOL WINAPI xmlDllMain(...
to BOOL XMLCALL xmlDllMain(
If anyone in some bizarre manner is somehow actually using this as it is 
right now, this change would probably break their usage.

Next - Right now the Makefile has to be manually edited to add the 
LIBXML_STATIC_FOR_DLL flag. I would like to add the ability to have this 
automatically performed by maybe a argument for configure script. This 
now brings up another question on whether the resulting library should 
still be called libxml2_a.lib or whether a third library should be 
produced to be able to distinguish between them (libxml2_a_dll.lib or 
something along those lines).

Personally I would like to see all 3 libs built by default and would 
like to hear anyone's opinion on this (especially since this would 
affect anyone distributing the binaries by the increase in size).

Rob