CMake — не существует правила для таргетинга

В Ubuntu я загрузил стороннюю общую библиотеку (liba1.so), размещенную в /lib/extern/lib. Связанные файлы заголовков я поместил в /lib/extern/include. Мои собственные заголовочные файлы размещены в /include/public/ и /include/private/. И теперь (чтобы протестировать mylib) я хочу связать это с моим кодом main.cpp, используя CMake.

Моя структура:

**My structure:** 
| 
| 
+---CMakeLists.txt 
| 
+---lib 
|    | 
|    +---extern 
|         |    
|         +---lib 
|         |    | 
|         |    +---liba.so 
|         |    +---libb.so 
|         |    +---liba12.so 
|         | 
|         +---include 
|                |     
|                +...headers.h 
|                
+---include 
|     | 
|     +---public 
|     |     | 
|     |     +---file1.hpp 
|     |     +... 
|     |
|     +---private 
|            | 
|            +---file2.hpp 
|            +... 
| 
+---src 
     | 
     +---public 
     |     | 
     |     +---file1.cpp 
     |     +... 
     | 
     +---private 
     |     | 
     |     +---file2.cpp 
     |     +...
     | 
     +---main.cpp

Мой CMakeLists.txt:

cmake_minimum_required(VERSION 3.9...3.19) 
if (${CMAKE_VERSION} VERSION_LESS 3.12) 
     cmake_policy(VERSION ${CMAKE_VERSION}) 
endif () 

project(mylib VERSION 0.0.1 DESCRIPTION "Test" LANGUAGES CXX ) 

set(CMAKE_CXX_FLAGS "${CMAKE_C_FLAGS} -DTPM_POSIX") 
set(CMAKE_CXX_STANDARD 20) set(CMAKE_CXX_STANDARD_REQUIRED ON) 
set(CMAKE_CXX_EXTENSIONS OFF) 

add_library(${PROJECT_NAME} STATIC src/private/file2.cpp src/public/file1.cpp ) 
add_library(lib SHARED IMPORTED) 

set_target_properties(lib PROPERTIES IMPORTED_LOCATION ${CMAKE_CURRENT_BINARY_DIR}/lib/extern/lib/liba.so) 

set_target_properties(lib PROPERTIES IMPORTED_LOCATION ${CMAKE_CURRENT_BINARY_DIR}/lib/extern/lib/libb.so) 

set_target_properties(lib PROPERTIES IMPORTED_LOCATION ${CMAKE_CURRENT_BINARY_DIR}/lib/extern/lib/liba12.so) 

target_link_libraries(${PROJECT_NAME} PUBLIC lib) 

target_include_directories(${PROJECT_NAME} 
   PUBLIC 
     $<INSTALL_INTERFACE:lib/extern/include>
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/lib/extern/include>

     $<INSTALL_INTERFACE:include/public>
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include/public>

     $<INSTALL_INTERFACE:include/private>
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include/private> 
) 

add_executable(test src/main.cpp) 

target_link_libraries(test PRIVATE mylib) 

ERROR: 

[build] Consolidate compiler generated dependencies of target mylib 
[build] [ 60%] Built target mylib [build] Consolidate compiler generated dependencies of target test 
[build] gmake[2]: *** No rule exists to target „lib/extern/lib/liba12.so“, 
[build] required by „test“ to create. Ending. 
[build] gmake[1]: *** [CMakeFiles/Makefile2:111: CMakeFiles/test.dir/all] Error 2 
[build] gmake: *** [Makefile:91: all] Error 2 
[proc] The command: /usr/bin/cmake --build /home/mathew/proj/build --config Debug --target all -j 10 -- exited with code: 2 and signal: null [build] Build finished with exit code 2