COMP: Add CMAKE_TOOLCHAIN_FILE to external project init cache
COMP: Add CMAKE_TOOLCHAIN_FILE to external project init cache

This commit adds support for cross-compilation with e.g. mxe.
kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix,kaspermarstal/SimpleElastix