Clang warning: linker input unused

Question

I have this warning message from clang when compiling a C++ project using Jansson JSON library.

clang: warning: -ljansson: 'linker' input unused

I'd like to get rid of it but I really have no idea what causes this.

Here is the CMake in question

project(sample)

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -ljansson")

set(SOURCE_FILES main.cpp)

add_executable(sample ${SOURCE_FILES})