Looks like Debian 9 CMake version is old. According to CMake documentation OBJECT library can be used in target_link_libraries since 3.12
CMake Error at src/lib/small/test/CMakeLists.txt:9 (target_link_libraries):
Target "small_unit" of type OBJECT_LIBRARY may not be linked into another
target. One may link only to STATIC or SHARED libraries, or to executables
with the ENABLE_EXPORTS property set.
Looks like Debian 9 CMake version is old. According to CMake documentation OBJECT library can be used in target_link_libraries since 3.12