[xml] wrong compiler settings for MSVC in makefile

[<oliverst online de>]

I tried to link the libxml2 statically into an application, but I got
some error messages while trying to link it. It said some symbols were
already defined in LIBCMT.lib. The problem are different runtime library
setting of the lib and the application I am trying to link it in. I took
a look at the makefiles of the libxml2 and recongised it always compiled
it with "/MD", which would be teh "Multithreaded DLL" runtime library.
That's OK, if you compile the libxml2.dll file and the non-static
library. But it's bad for the static lib. So I case of teh static lib it
should be set to "/MT" (Multithreaded), if "threads" aren't set to "no"
or "/ML" if "threads" are set to "no" in the configure.js.
Unfortunately I don't have a diff/patch for it, because I am not quite
sure how to handle different CFLAGS for the static and non-static lib in
the makefile, because they used to use the same.