[CMake] target_link_libraries fails dependencies

[Andrea Galeazzi]

I'm using cmake_policy(SET CMP0015 NEW) to link external libraries (I 
don't have the sources of them) in order to avoid to specify the 
absolute path of each library. So I wrote the following commands:
link_directories(path1 path2 ...)
target_link_libraries(${my_TARGET_NAME} lib1 lib2)

but when lib1 or lib2 changes, my_TARGET_NAME doesn't rebuild the 
executable. On the other hand if I use the absolute path in 
target_link_libraries (path1/lib1.a) everything works fine.
Is this behavior a bug of CMake or is not possible to detect this kind 
of dependencies at all?