"BUG: Prefix USE_SYSTEM_ELASTIX with SimpleITK_"
"BUG: Prefix USE_SYSTEM_ELASTIX with SimpleITK_"

SimpleITK updated its naming scheme for v1.0.0. This brings SimpleITK_USE_SYSTEM_ELASTIX on par with the Travis CI configuration and the rest of the build system.

kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix