[CMake] Triggering individual steps of externalproject_add add cmake time

[Jan Wurster]

 Dear list, 

 as I'm encountering a case where I'd like to trigger the download of a
file (3rd party) structure from a git repository I've been looking into
ExternalProject_add and its benefits. However the key element would be
to perform the download step immediately, i.e. at cmake first run time
in the configure step of the wrapping project.

 Is it possible to trigger individual steps to be done 'now' instead of
during build time or should I look at implementing a custom script for
this case? 

 Thanks in advance, cheers,
Jan