ExternalProject is not well suited in superbuild projects
We are thinking to switch from using add_subdirectory for our superbuild to ExternalProject. This has the benefit that we can define the same target more than once, if it is not exported in the same namespace, plus more separation of components that need to be explicit about their dependencies.
However, at the moment we have no way to forward reliably CMake command line options, and all the cached variables ("cmake -C" for some strange reason does not accept a cache file as input).
This is a major hindrance, as we have lots and lots variables which are regularly overridden on the command line or through their IDE by users, and they change relatively often as the project grows.
We need a way to pass all cached and command line variables to children instances of CMake.