And iff you're building against a static libxml2 library, you need to define -DLIBXML_STATIC. So the Makefile.PL would finally look like:

use ExtUtils::MakeMaker;

WriteMakefile(
    NAME        => 'XML::LibXML::Common',
    VERSION_FROM    => 'Common.pm', # finds $VERSION
    AUTHOR        => 'Christian Glahn <christian.glahn@uibk.ac.at>',
    ABSTRACT      => 'Routines and Constants common for XML::LibXML an
+d XML::GDOME',
    DEFINE        => '-DLIBXML_STATIC',
    INC           => "-Ic:/libxml2/include",
    LIBS          => [ "-Lc:/libxml2/lib -lxml2", ],
);
[download]

(But leave that DEFINE out if you want to build against a dll.)

Cheers,
Rob