[CMake] How to export the user-defined variable in to the parent directory?

[黄光成]

my directory hierarchical is : dir1/dir2/dir3. 
In dir3's CMakeLists.txt, I define a custom variable named 'xxx' using the command set(xxx yyy). 
I want to use the variable 'xxx' in dir1's CMakeLists.txt, how to do that? Can export the user-defined variable?
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