Emit warning instead of failure if libxml2 not available
Emit warning instead of failure if libxml2 not available

pabigot/pyxb,CantemoInternal/pyxb,jonfoster/pyxb-upstream-mirror,balanced/PyXB,jonfoster/pyxb-upstream-mirror,jonfoster/pyxb2,balanced/PyXB,CantemoInternal/pyxb,pabigot/pyxb,balanced/PyXB,CantemoInternal/pyxb,jonfoster/pyxb2,jonfoster/pyxb2,jonfoster/pyxb-upstream-mirror