title: Unable to find the requested Boost libraries in cmake with conan
body: <p>I have a problem to compile with cmake using the boost library in c++ from the conan dependencies in a Unix System (Ubuntu)</p>
  
  <p>The sfml library is already implemented and working in the project but the boost library doesn't work and is displaying this message :</p>
  
  <pre><code>CMake Error at /usr/share/cmake-3.13/Modules/FindBoost.cmake:2100 (message):
    Unable to find the requested Boost libraries.
  
    Unable to find the Boost header files.  Please set BOOST_ROOT to the root
    directory containing Boost or BOOST_INCLUDEDIR to the directory containing
    Boost's headers.
  Call Stack (most recent call first):
    CMakeLists.txt:13 (find_package)
  </code></pre>
  
  <p>Here is my conanfile.txt</p>
  
  <pre><code>[requires]
  sfml/2.5.1@bincrafters/stable
  boost/1.69.0@conan/stable
  
  [options]
  sfml:graphics=True
  sfml:window=True
  sfml:audio=True
  sfml:network=False
  
  [generators]
  cmake
  </code></pre>
  
  <p>Here is my CMakeLists.txt</p>
  
  <pre><code># Minimum version of cmake
  cmake_minimum_required(VERSION 3.10)
  
  # Setting the project
  project(CPP_rtype_2019 CXX)
  include(build/conanbuildinfo.cmake)
  
  # Dependencies Sfml
  find_package(SFML 2.5.1 EXACT REQUIRED COMPONENTS system window graphics audio)
  find_package(Threads REQUIRED)
  
  # Dependencies Boost
  find_package(Boost 1.69.0 EXACT REQUIRED COMPONENTS thread system filesystem)
  include_directories(${Boost_INCLUDE_DIRS})
  
  # Set the C++11 standard
  set(CMAKE_CXX_STANDARD 11)
  
  # Added all .hpp
  include_directories(client/include)
  include_directories(server/include)
  
  # Added all .cpp
  add_executable(r-type_client client/src/Actor.cpp client/src/Character.cpp client/src/Ennemy.cpp client/src/Interface.cpp client/src/Pown.cpp client/src/Projectile.cpp)
  add_executable(r-type_server server/src/GameEngine.cpp)
  
  # Added dependencies to compilation
  target_link_libraries(r-type_client PRIVATE sfml-audio sfml-system sfml-graphics)
  target_link_libraries(r-type_server PRIVATE Boost::thread Boost::system Boost::filesystem)
  </code></pre>
  
  <p>I've tried so many solutions but no one is working. All solutions or tips are welcome, thanks !</p>