CMake 3.13 is not present in older (~2 years old)
Linux distributions and GNU Radio requires min CMake
All that is needed in order to avoid bumping CMake version
is to not use 'target_link_libraries' in subdirectories.
Here this is done by creating a list of needed
libraries and adding them for linking at the end (like
it was done in gr-osmosdr before porting to GNU Radio 3.8).
One thing that is lost here is 'PRIVATE' statement in case
of FCD libraries linking.
Signed-off-by: Eric Wild <email@example.com>