CMake cannot link executable -ljsoncpp: no such file using github submodules - c++

Question

I am working in a project which uses jsoncpp for parsing and cmake for compilation. I added the jsoncpp official git repository as a submodule to my project with git submodule add REPO_URL external/jsoncpp, so as to keep every dependency together.
When running cmake -B out/build, it works normally. But when I do make, I get the following error:
/usr/bin/ld: cannot find -ljsoncpp: No such file or directory.
The files are organized the following way:
- root
- out/build
- external
- jsoncpp (cloned repo)
- include
foo.h
bar.h
- src
foo.cpp
bar.cpp
main.cpp
CMakeLists.txt
The CMakeLists.txt is like this:
cmake_minimum_required(VERSION 3.22.1)
project(ants)
# ".cpp" files in folder "src" into cmake variable "SOURCE"
file(GLOB SOURCE "src/*.cpp")
# Executable
add_executable(${PROJECT_NAME} ${SOURCE})
# Directory where cmake will look for include files
include_directories(include)
# Tells cmake to compile jsoncpp
add_subdirectory(external/jsoncpp)
# Tells cmake where to look for jsoncpp include files
target_include_directories(${PROJECT_NAME}
PUBLIC external/jsoncpp/include
)
target_link_libraries(${PROJECT_NAME} jsoncpp)

Answer 1

The jsoncppConfig.cmake defines property INTERFACE_INCLUDE_DIRECTORIES for targets jsoncpp_lib and jsoncpp_lib_static.
You need to query the target property and set it manually:
get_target_property(JSON_INC_PATH jsoncpp_lib INTERFACE_INCLUDE_DIRECTORIES)
include_directories(${JSON_INC_PATH})
Linking is done via:
target_link_libraries(${PROJECT_NAME} jsoncpp_lib)
Source.
Try this:
cmake_minimum_required(VERSION 3.22.1)
project(ants)
# ".cpp" files in folder "src" into cmake variable "SOURCE"
file(GLOB SOURCE "src/*.cpp")
# Executable
add_executable(${PROJECT_NAME} ${SOURCE})
# Directory where cmake will look for include files
include_directories(include)
# Tells cmake to compile jsoncpp
add_subdirectory(external/jsoncpp)
get_target_property(JSON_INC_PATH jsoncpp_lib INTERFACE_INCLUDE_DIRECTORIES)
include_directories(${JSON_INC_PATH})
target_link_libraries(${PROJECT_NAME} jsoncpp_lib)