Correct capitalisation of LibXml2_FOUND  variable.
Correct capitalisation of LibXml2_FOUND  variable.

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