CMake: undefined reference to

I use CMake 3.5.2. When trying to build my C++ code I get the following error:

[100%] Linking CXX executable SomeExecutable
CMakeFiles/http://ift.tt/28UhxWz: In function `FunctionOne::FunctionOne(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, double, double, unsigned int, bool)':
FunctionOne.cpp:(.text+0x490): undefined reference to `Helper::initLevel(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bool)'

How do I fix it?

CMakeLists.txt: (shortened for brevity)

cmake_minimum_required(VERSION 3.0)
project(ProjectName)

add_definitions(-std=c++11)
add_definitions(-Wall)
add_definitions(-O2)

link_directories(/usr/local/lib)    

add_executable(SomeExecutable SomeExecutable.cpp ${OptFunctionOne} ${OptFunctionTwo})
target_link_libraries(SomeExecutable -pthread -lboost_thread ${Boost_LIBRARIES} myLib armadillo)

File structure (shortened for brevity):

├── bin
├── CMakeCache.txt
├── cmake_install.cmake
├── CMakeLists.txt
├── Common
│   ├── include
│   │   ├── dataContainer.h
│   │   ├── OptFunctionOne.h
│   │   ├── OptFunctionTwo.h
│   └── src
│       ├── OptFunctionOne.cpp
│       └── OptFunctionTwo.cpp
├── SomeExecutable.cpp
├── myLib
│   ├── example.cpp
│   ├── include
│   │   ├── Config.h
│   │   ├── Helper.h
│   ├── libmyLib.so