[CMake] Which variable stores all (sub) directories added sofar?

[J.S. van Bethlehem]

Hello,

Given that CMake errs when you try to some sub-directory twice using 
add_subdirectory() I presume there is some variable that stores a list 
of the directories that were added to the build sofar. But I can't find 
the name of the variable. Does it exists (well... better would be: is it 
visible to users) and if so, what is it called?

Yours sinceryl,
Jakob