Linking error while using FFTW as backend for the Eigen FFT implementation

Question

I'm currently attempting to use FFTW as the backend for Eigen's FFT implementation, which is located in the unsupported section of Eigen. However, I've encountered a compilation error due to linker issues.

In a nutshell, my project has the following directory structure:

project_root
│   CMakeLists.txt
└───src
│   └───ccs
│       │   CMakeLists.txt
│       │   LDPCDecoder.h
│       │   LDPCDecoder.cpp
└───thirdparty
    └───fftw
        └───...

I'm utilizing Eigen's FFT in LDPCDecoder.cpp.

During compilation, I'm encountering numerous "undefined reference" errors like the following:

[build] LDPC_decoder.cpp:(.text+0x8fa0): undefined reference to `fftwf_execute_dft_r2c'
[build] /usr/bin/ld: LDPC_decoder.cpp:(.text+0x93b1): undefined reference to                fftwf_execute_dft_c2r'
[build] /usr/bin/ld: LDPC_decoder.cpp:(.text+0x9a28): undefined reference to fftwf_plan_dft_r2c_1d'
[build] /usr/bin/ld: LDPC_decoder.cpp:(.text+0x9d7e): undefined reference to fftwf_plan_dft_c2r_1d'
[build] collect2: error: ld returned 1 exit status`

I've installed the FFTW library in the thirdparty/fftw directory using the following steps:

./configure --prefix <PathTofftwDirectory>
make
make install

As a result, I now have 'libfftw3.a' located in 'thirdparty/fftw/lib/libfftw3.a.'

Here's the content of the CMakeLists.txt file in the ccs directory:

add_library(ccs
    tree_decoder.cpp
    tree_decoder.h
    tree_encoder.cpp
    tree_encoder.h
    treecode_generation.h
    treecode_generation.cpp
    ccs_encoder.h
    ccs_decoder.h
    SPARCS_encoder.h
    SPARCS_encoder.cpp
    SPARCS_decoder.h
    SPARCS_decoder.cpp
    tree_encoder_BPSPARCS.h
    tree_encoder_BPSPARCS.cpp
    BPSPARCS_encoder.h
    BPSPARCS_encoder.cpp
    BPSPARCS_decoder.h
    BPSPARCS_decoder.cpp
    LDPC_decoder.h
    LDPC_decoder.cpp
    tree_decoder_BPSPARCS.h
    tree_decoder_BPSPARCS.cpp
    tree_decoder_triadicGraph.h
    tree_decoder_triadicGraph.cpp
    BPSPARCS_codingParameters.h
    BPSPARCS_codingParameters.cpp
)

target_include_directories(ccs
    PUBLIC
    ${CMAKE_CURRENT_SOURCE_DIR}
    ${CMAKE_SOURCE_DIR}/thirdparty/aff3ct
    ${CMAKE_SOURCE_DIR}/thirdparty/fftw/include
)

link_directories(ccs ${CMAKE_SOURCE_DIR}/thirdparty/fftw/lib)

target_link_libraries(ccs
    ${CMAKE_SOURCE_DIR}/thirdparty/fftw/lib/libfftw3.a
    aff3ct-static-lib
)

Does anyone have any ideas on how to resolve these linker errors?

I tried to link the library againts the toplevel executable instead ob the static lib ccs, but this did not fix the error. I also did a clean rebuild of the project and a rebuild of the fftw3lib.a