Q:

CMake link directory passing when compiling shared library

Say I have C project with the following structure (simplified):
|- CMakeLists.txt   <- This is root CMake
|- lib
    |- <some source files>
    |- CMakeLists.txt   <- CMake file for building the library
|- demo
    |- <some source files>
    |- CMakeLists.txt   <- CMake for building demo apps
|- extra_lib
    |- <some source files>
    |- CMakeLists.txt   <- CMake for building supplementary library

Now, I want to build my library (living in lib) as a shared library to be used by demo apps from demo directory. 
Additional library, that can not be a part of my library (it is essentially a wrapper for some C++ external library) is also to be compiled as a shared library and then linked to my library.
I have a problem with including dependencies for additional library. In its CMakeLists.txt I've defined link_directories to point location where .so libs are stored and then target_link_libraries to point which should be linked. At the end I did export target.
include_directories(${EXTERNAL_DIR}/include)
link_directories(${EXTERNAL_DIR}/lib)

add_library(extra_lib SHARED extra_lib.cpp)

target_link_libraries(extra_lib
  some_lib
)

export(TARGETS extra_lib FILE extra_lib.cmake)

The point is that when I try to compile lib and link it against extra_lib I get an error that some_lib is not found what I guess means that link_directories is local to the extra_lib.
Now, question is how can I make it propagate together with dependencies? I'd like it to work in the way that adding extra_lib as subdirectory and as a dependency for my lib would automatically add linked directories from extra_lib to the lib linking process.
The linking process would look like:
(some external library) --> extra_lib --> lib --> demo app

A:

First off, the CMake docs state that commands like include_directories and link_directories are rarely necessary. In fact, it is almost always better to use target_include_directories and target_link_libraries instead.
Secondly, the reason your approach fails is because you need to let CMake know about the existence of some_lib. You can do this like so:
add_library(some_lib SHARED IMPORTED)
set_target_properties(some_lib
    PROPERTIES
        IMPORTED_LOCATION ${EXTERNAL_DIR}/lib/libsome_lib.so)

Then, afterwards:
target_link_libraries(extra_lib some_lib)

