Hide the mess of cache variables before finding libXml2.
Hide the mess of cache variables before finding libXml2.

hsorby/libcellml,cellml/libcellml,nickerso/libcellml,cellml/libcellml,hsorby/libcellml,nickerso/libcellml,nickerso/libcellml,hsorby/libcellml,cellml/libcellml,cellml/libcellml,MichaelClerx/libcellml,hsorby/libcellml,nickerso/libcellml,MichaelClerx/libcellml,MichaelClerx/libcellml