CMake add dependencies to internal library in a cousin directory - c++

Question

I have following project structure:
│ CMakeLists.txt
├───lib
│ │ CMakeLists.txt
│ ├───lib_a
│ │ CMakeLists.txt
│ ├───lib_b
│ │ CMakeLists.txt
│ └───lib_c
│ CMakeLists.txt
└───src
│ CMakeLists.txt
└───app
├───controller
| CMakeLists.txt
└───endorser
CMakeLists.txt
The libraries are located in lib directory. lib_b links against lib_a and lib_c links against lib_b. CMake recognizes targets between libraries with no problem. For example having these lines in lib/lib_c/CMakeLists.txt file works perfectly fine:
target_link_libraries(lib_c lib_a lib_b)
However trying to do the same in the src/app/controller fails with following error:
[cmake] CMake Error at src/app/controller/CMakeLists.txt:15 (find_package):
[cmake] By not providing "Findlib_c.cmake" in CMAKE_MODULE_PATH this project has
[cmake] asked CMake to find a package configuration file provided by "lib_c", but
[cmake] CMake did not find one.
[cmake]
[cmake] Could not find a package configuration file provided by "lib_c" with any of
[cmake] the following names:
[cmake]
[cmake] lib_cConfig.cmake
[cmake] lib_c-config.cmake
And this is how I search for lib_c:
# Check for dependencies
find_package(lib_c REQUIRED)
...
# link against it
target_link_libraries(controller lib_c)
How can I export lib_c without having to install it, in such a way that src/app can link against?
Anyway, CMakeLists.txt of non-leaf directories contain only add_subdirectory calls:
# lib/CMakeLists.txt
cmake_minimum_required(VERSION 3.1.0)
add_subdirectory(lib_a)
add_subdirectory(lib_b)
add_subdirectory(lib_c)