Fix to build pugixml with older cmake version
Fix to build pugixml with older cmake version

wbinventor/openmc,liangjg/openmc,walshjon/openmc,bhermanmit/openmc,walshjon/openmc,shikhar413/openmc,johnnyliu27/openmc,amandalund/openmc,liangjg/openmc,shikhar413/openmc,smharper/openmc,paulromano/openmc,smharper/openmc,wbinventor/openmc,shikhar413/openmc,smharper/openmc,liangjg/openmc,amandalund/openmc,mit-crpg/openmc,paulromano/openmc,walshjon/openmc,bhermanmit/openmc,johnnyliu27/openmc,liangjg/openmc,wbinventor/openmc,mit-crpg/openmc,johnnyliu27/openmc,mit-crpg/openmc,mit-crpg/openmc,shikhar413/openmc,amandalund/openmc,amandalund/openmc,paulromano/openmc,paulromano/openmc,wbinventor/openmc,johnnyliu27/openmc,walshjon/openmc,smharper/openmc