title: CMake FIND_LIBRARY: link to specified library error
body: <p>I have already installed the FFTW3 library on my computer and the following files could be found in /usr/lib:</p>
  
  <pre><code>libfftw3f.so.3          libfftw3l_threads.so.3
  libfftw3f.so.3.3.0      libfftw3l_threads.so.3.3.0
  libfftw3f_threads.so.3      libfftw3.so.3
  libfftw3f_threads.so.3.3.0  libfftw3.so.3.3.0
  libfftw3l.so.3          libfftw3_threads.so.3
  libfftw3l.so.3.3.0      libfftw3_threads.so.3.3.0
  </code></pre>
  
  <p>I want to install another package that needs to link these libraries, but when I try 
  <code>FIND_LIBRARY(FFTW3_LIBRARIES fftw3)</code> and <code>FIND_LIBRARY(FFTW3_LIBRARIES fftw3f)</code>, 
  it just cannot find the libraries.</p>
  
  <p>How can I solve this? Thanks!</p>
  
  <p>Code in CMakeLists.txt:</p>
  
  <pre><code>FIND_PATH(FFTW3_INCLUDE_DIR fftw3.h)
  IF(FFLD_HOGPYRAMID_DOUBLE)
    FIND_LIBRARY(FFTW3_LIBRARIES libfftw3.so.3)
  ELSE()
    FIND_LIBRARY(FFTW3_LIBRARIES libfftw3f.so.3)
  ENDIF()
  #IF(NOT FFTW3_INCLUDE_DIR OR NOT FFTW3_LIBRARIES)
  IF(NOT FFTW3_INCLUDE_DIR OR NOT FFTW3_LIBRARIES)
    MESSAGE(FATAL_ERROR "Could not find fftw3.")
  ENDIF()
  </code></pre>
  
  <p>error message:</p>
  
  <pre><code>CMake Error at CMakeLists.txt:52 (MESSAGE):
    Could not find fftw3.
  </code></pre>