[CMake] How to define a global scope variable ?

[Xiangyun Kong]

How could I define a global scoped variable in CMAKE, so that I could use
something like,

    set (global_name ${global_name} {files})

to collect all files in each sub-catogory recursively ?

It seems I could only use PARENT_SCOPE to make the parent directory
aware of the variable.

Thanks for any hints.

Xiangyun