[CMake] How to exclude linking from add_subdirectory?

[kipade]

In my cmake project, there are some subdirectories within it, and some of them have to build as completed 
static or shared libraries.
1. The top project is executable 
2. a subdirectory is a shared project, which also have some subdirectoy compile as static libraries which
   of course should ONLY link into their parent shared library
I add link options to linking the shared library produced by its subdirectory(which linked as a shared library), 
but I found the all sub-static library with the shared library project(the subdirectory) also double linked into
the top executable project. Of course I only want to link the shared library. How to resolve such problem 
for the top CMakeLists.txt?
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