MSVC build broken

The recent xml2 build changes have broken MSVC builds.

The problem is this fragment (that appears twice) from
src/tools/msvc/Mkvcbuild.pm:

foreach my $o (split /\s+/, $1)
{
$o =~ s/\.o$/.c/;
$proj->AddFile('contrib\\' . $n . '\\' . $o);
}

which gets applied to this line: (after "OBJS = " has been stripped):

OBJS = $(if $(filter -lxslt, $(LIBS)), xpath.o xslt_proc.o, xpath.o)

Our logic above is far too rough for such lines. I'll try to come up
with something more robust tomorrow.

cheers

andrew