Patch: Support libxml2 under MinGW32

Attached is a patch against head that allows MinGW32 to link against the
libxml2 dll instead of requiring the expat peer. expat is still the
default for MinGW32, but if expat is disabled via ABI_OPT_PEER_EXPAT=0,
we attempt to use libxml2 instead of the old (probably non-functional)
MSXML parser. MSXML can still be used by setting ABI_OPT_MSXML=1. Can
someone look over this before I commit and make sure I'm not borking
anything on the Linux side with this? Builds and works fine on win32.

Mikey