[CMake] ExternalProject with toolchain files for cross compiling?

[Robert Dailey]

So I'd like to use ExternalProject_Add() to pull down an external git
repository that builds with CMake. I'd like it to use the same
toolchain file that I"m using in my parent projects. However, I do not
see any explicit documentation regarding toolchain support for
ExternalProject. Do I need to explicitly pass down CMake arguments
with the toolchain file? Or does this happen transitively? I'm using
CMake 3.9.0