title: Inlcude fftw3.h with cmake in a linux environment
body: <p>I want to use fftw library.</p>
  <p>In my cpp file I included as <code>#include &lt;fftw3.h&gt;</code></p>
  <p>I downloaded the library and I saved it in ./fftw
  Then I typed:</p>
  <pre><code>cd fftw
  ./configure
  make
  make install
  cd ..
  </code></pre>
  <p>In my project I am already using OpenCV, so my CMakeLists.txt is:</p>
  <pre><code>cmake_minimum_required(VERSION 2.8) # CMake version check
  project( main ) 
  find_package( OpenCV REQUIRED )
  include_directories( ${OpenCV_INCLUDE_DIRS} )
  add_executable( main main.cpp )
  # set(CMAKE_BUILD_TYPE Debug)
  target_link_libraries( main ${OpenCV_LIBS} )
  target_link_libraries( main ./fftw)
  </code></pre>
  <p>Finally I execute <code>cmake CMakeLists.txt</code> but when I try to <code>make</code> my executable, it complains that fftw3.h: No such file or directory.</p>
  <p>How could I make it work?</p>
  <p>Thanks.</p>