CMAKE question: How to link a shared public library with all the targets in subdirectories

Question

I've got a PROJECT with many subdirectories, where the targets (executables) are listed. All the folders, the main one and the subfolders, have CMakeList.txt. I need a specify a particular library to be used when building and - especially - when linking all the targets. Targets are specified only in subfolders; I do not want to change the CMakeLists.txt in the subfoloders. Could I set and how do I set the public library in the top CMakeLists.txt, to be used with all the targets in subfolders?

Schematic view of the code:

CMakeLists.txt
code0.f
|------- subdir1
|-------------- CMakeLists.txt
|------------- code1.f
|------- subdir2
|------------- CMakeLists.txt
|------------- code2.f

The library from another location, say, /usr/mylib64/libstdc++.so.6 needs to be included in all the targets listed in subdir1 and subdir2 CMakeLists.txt files.

It works OK if it is added as

target_link_libraries(target_subdir1 
        PUBLIC /usr/mylib64/libstdc++.so.6

and

target_link_libraries(target_subdir2 
        PUBLIC /usr/mylib64/libstdc++.so.6

I need a similar outcome, but to set this library in the top-level CMakeLists.txt It does not have a target executable but it has

add_custom_target(build DEPENDS sorc/pbuild)

Will be happy to get any suggestions on how to solve it!