[CMake] Passing values back from SUBDIRS

[John Biddiscombe]

For each subdirectory traversed, I want to add some data into a 
variable, and then from the root directory, write out this data via the 
configure_file command with variable substitution (not IMMEDIATE file 
write)

I can't get it to work. (Perhaps I could write data to a file using 
append and then read it back somehow)

Is there a correct way to pass data from subdirs back to parent dirs?

thanks

JB