cmake linking library issue - c++

Question

Hello and thanks in advance for the help,
I have a linking issue with my cmake that I don't get. I'm trying to include a shared lib(modbuspp) in submodule which require another shared lib(libmodbus) on my computer. It seem simple but during exec linking it give me undefined reference to my computer lib (libmodbus) when I did add the includes. Does anyone has an idea why ? Here is my Cmake:
cmake_minimum_required(VERSION 3.10)
set(CMAKE_BUILD_TYPE Release)
set(CMAKE_CXX_STANDARD 14)
set(CMAKE_POSITION_INDEPENDENT_CODE ON)
set(This test)
project(${This} VERSION "0.1.0")
configure_file("${PROJECT_SOURCE_DIR}/src/config.h.in" "${PROJECT_SOURCE_DIR}/src/config.h")
#enable_testing()
#add_subdirectory(googletest)
#add_subdirectory(test)
file(GLOB Sources
"src/*.hpp"
"src/*.cpp"
)
add_executable(${This} ${Sources})
find_package(nlohmann_json 3.2.0 REQUIRED)
set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/Modules/")
find_package(Modbus REQUIRED)
message(TROLOLO:${MODBUS_INCLUDE_DIRS})
include_directories(${MODBUS_INCLUDE_DIRS})
#target_include_directories(${This} PRIVATE ${MODBUS_INCLUDE_DIRS})
#work on libmodbuspp exemple "-shared -fPIC -pthread -I/usr/local/include -I/usr/include/modbus -L/usr/local/lib -lmodbuspp -lmodbus"
#Configure: cmake -DMODBUSPP_USE_EXTERNAL_JSON=ON -DMODBUSPP_WITH_STATIC=1 -DINSTALL_LIB_DIR=libs -DMODBUSPP_UNIT_TESTS=0 -S . -B build
add_subdirectory(external/libmodbuspp)
include_directories(external/libmodbuspp/include)
#target_include_directories(${This} PRIVATE external/libmodbuspp/include)
link_directories(${This} PRIVATE ${PROJECT_BINARY_DIR}/external/libmodbuspp/lib)
target_link_libraries(${This} PRIVATE ${MODBUS_LIBRARIES} modbuspp-shared)
set(THREADS_PREFER_PTHREAD_FLAG ON)
find_package(Threads REQUIRED)
target_link_libraries(${This} PUBLIC Threads::Threads)
And here are the compile error:
Scanning dependencies of target objlib
[ 4%] Building CXX object external/libmodbuspp/lib/CMakeFiles/objlib.dir/__/src/bufferedslave.cpp.o
[ 9%] Building CXX object external/libmodbuspp/lib/CMakeFiles/objlib.dir/__/src/device.cpp.o
[ 14%] Building CXX object external/libmodbuspp/lib/CMakeFiles/objlib.dir/__/src/master.cpp.o
[ 19%] Building CXX object external/libmodbuspp/lib/CMakeFiles/objlib.dir/__/src/message.cpp.o
[ 23%] Building CXX object external/libmodbuspp/lib/CMakeFiles/objlib.dir/__/src/netlayer.cpp.o
[ 28%] Building CXX object external/libmodbuspp/lib/CMakeFiles/objlib.dir/__/src/request.cpp.o
[ 33%] Building CXX object external/libmodbuspp/lib/CMakeFiles/objlib.dir/__/src/response.cpp.o
[ 38%] Building CXX object external/libmodbuspp/lib/CMakeFiles/objlib.dir/__/src/router.cpp.o
[ 42%] Building CXX object external/libmodbuspp/lib/CMakeFiles/objlib.dir/__/src/rtulayer.cpp.o
[ 47%] Building CXX object external/libmodbuspp/lib/CMakeFiles/objlib.dir/__/src/server.cpp.o
/home/aurelia/git/test/external/libmodbuspp/src/server.cpp: In static member function ‘static void* Modbus::Server::Private::loop(std::future<void>, Modbus::Server::Private*)’:
/home/aurelia/git/test/external/libmodbuspp/src/server.cpp:434:3: warning: no return statement in function returning non-void [-Wreturn-type]
434 | }
| ^
[ 52%] Building CXX object external/libmodbuspp/lib/CMakeFiles/objlib.dir/__/src/slave.cpp.o
[ 57%] Building CXX object external/libmodbuspp/lib/CMakeFiles/objlib.dir/__/src/tcplayer.cpp.o
[ 61%] Building CXX object external/libmodbuspp/lib/CMakeFiles/objlib.dir/__/src/timeout.cpp.o
[ 61%] Built target objlib
Scanning dependencies of target modbuspp-shared
[ 66%] Linking CXX shared library libmodbuspp.so
[ 66%] Built target modbuspp-shared
Scanning dependencies of target test
[ 71%] Building CXX object CMakeFiles/test.dir/src/StaticDI.cpp.o
[ 76%] Building CXX object CMakeFiles/test.dir/src/StaticDO.cpp.o
[ 80%] Building CXX object CMakeFiles/test.dir/src/conf.cpp.o
[ 85%] Building CXX object CMakeFiles/test.dir/src/main.cpp.o
[ 90%] Building CXX object CMakeFiles/test.dir/src/modbusServer.cpp.o
[ 95%] Linking CXX executable test
/usr/bin/ld: external/libmodbuspp/lib/libmodbuspp.so.1.0.0: undefined reference to `modbus_rtu_set_rts_delay'
/usr/bin/ld: external/libmodbuspp/lib/libmodbuspp.so.1.0.0: undefined reference to `modbus_get_byte_timeout'
/usr/bin/ld: external/libmodbuspp/lib/libmodbuspp.so.1.0.0: undefined reference to `modbus_rtu_get_rts'
/usr/bin/ld: external/libmodbuspp/lib/libmodbuspp.so.1.0.0: undefined reference to `modbus_close'
/usr/bin/ld: external/libmodbuspp/lib/libmodbuspp.so.1.0.0: undefined reference to `modbus_mapping_free'
/usr/bin/ld: external/libmodbuspp/lib/libmodbuspp.so.1.0.0: undefined reference to `modbus_set_indication_time
But all those reference can be found in the include that was linked with include_directories :
/usr/include/modbus$ grep rtu_set_rts_delay ./*
./modbus-rtu.h:MODBUS_API int modbus_rtu_set_rts_delay(modbus_t *ctx, int us);