title: Can not find fftw3.h even after adding them to PATH
body: <h2>Basis</h2>
  <p>I have installed FFTW in my system manually by using <code>./configure</code>, <code>make</code> commands the procedure mentioned in the <a href="http://www.fftw.org/fftw2_doc/fftw_6.html" rel="nofollow noreferrer">website</a> using some flags.</p>
  <p>I do thus have fftw3.h header file as can be seen below</p>
  <pre class="lang-sh prettyprint-override"><code>locate fftw3.h
  /home/anirbankopty/Softwares/FFTW/fftw-3.3.10/api/fftw3.h
  /home/anirbankopty/Softwares/FFTW/fftw-install/include/fftw3.h
  </code></pre>
  <h2>Problem</h2>
  <p>But, still, when compiling my <code>C</code> code using the <code>-lfftw3</code> flag, it says</p>
  <pre class="lang-sh prettyprint-override"><code>gcc FFT_denoise.c -lfftw3 -lm
  FFT_denoise.c:5:10: fatal error: fftw3.h: No such file or directory
      5 | #include &lt;fftw3.h&gt;
        |          ^~~~~~~~~
  compilation terminated.
  </code></pre>
  <p>I tried doing in <code>Fortran</code> and also there I am getting</p>
  <pre class="lang-sh prettyprint-override"><code>gfortran FFT_denoise.f03 -lfftw3 -lm
  /usr/bin/ld: cannot find -lfftw3
  collect2: error: ld returned 1 exit status
  </code></pre>
  <h2>Tried</h2>
  <p>I Tried adding those fftw3.h path i.e.
  <code>/home/anirbankopty/Softwares/FFTW/fftw-install/include/</code> to the PATH variable, but still the problem persists.</p>
  <p>I use zsh shell.</p>