title: problems with cmake and boost-signals
body: <p>I'm using cmake for project with boost, I've set boost-signals as required and cmake states it finds it, but when I compile the project I get a linking error.</p>
  
  <p>I can resolve it with a linker directive  set(CMAKE_EXE_LINKER_FLAGS -lboost_signals) but 
  this seems contrary to what it should be doing. Can someone suggest a better way? Thanks</p>
  
  <p>in CMakeLists.txt</p>
  
  <pre><code>   IF(UNIX)
          find_package(Boost COMPONENTS system filesystem random regex signals thread program_options date_time REQUIRED)
  
              INCLUDE_DIRECTORIES( ${Boost_INCLUDE_DIRS} ) 
  
      ENDIF()
  </code></pre>
  
  <p>Running cmake:</p>
  
  <pre><code>   $ cmake ../
      -- Boost version: 1.49.0
      -- Found the following Boost libraries:
      --   system
      --   filesystem
      --   random
      --   regex
      --   signals
      --   thread
      --   program_options
      --   date_time
      -- Configuring done
      -- Generating done
      -- Build files have been written to: /home/me/myproject/build
  </code></pre>
  
  <p>Linker error:</p>
  
  <pre><code>/usr/bin/ld: /usr/local/lib/libwt.so: undefined reference to symbol '_ZN5boost7signals9trackableD2Ev'
  /usr/bin/ld: note: '_ZN5boost7signals9trackableD2Ev' is defined in DSO /usr/lib/libboost_signals.so.1.49.0 so try adding it to the linker command line
  /usr/lib/libboost_signals.so.1.49.0: could not read symbols: Invalid operation
  collect2: error: ld returned 1 exit status
  </code></pre>