title: Doxygen - Could NOT find FLEX (missing: FLEX_EXECUTABLE)
body: <p>I know there are very similar worded questions on here, but I could not find an answer to my question there, so here we go:<br>
  I'm trying to see which of my C++ methods are called by others so I found Doxygen after googling.<br>
  On their <a href="http://www.doxygen.nl/download.html" rel="noreferrer">page</a> the installation seems pretty straightforward:  </p>
  
  <blockquote>
    <p>If you have the necessary build tools installed (i.e. g++, python,
    cmake, flex, bison), you should do the following to get the initial
    copy of the repository:</p>
    
    <p><code>git clone https://github.com/doxygen/doxygen.git</code><br>
    <code>cd doxygen</code><br>
    <code>mkdir build</code><br>
    <code>cd build</code><br>
    <code>cmake -G "Unix Makefiles" ..</code><br>
    <code>make</code></p>
  </blockquote>
  
  <p>Until <code>cmake -G "Unix Makefiles" ..</code> everything goes well, then on that command following error occurs: </p>
  
  <pre><code>test@test-VirtualBox:~/doxygen/build$ cmake -G "Unix Makefiles" ..
  CMake Error at /usr/share/cmake-3.5/Modules/FindPackageHandleStandardArgs.cmake:148 (message):
    Could NOT find FLEX (missing: FLEX_EXECUTABLE)
  Call Stack (most recent call first):
    /usr/share/cmake-3.5/Modules/FindPackageHandleStandardArgs.cmake:388 (_FPHSA_FAILURE_MESSAGE)
    /usr/share/cmake-3.5/Modules/FindFLEX.cmake:230 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
    CMakeLists.txt:84 (find_package)
  </code></pre>
  
  <p>I thought: <em>No big deal</em>, why should anything work out of the box...seems I have to install <code>flex</code>.<br>
  So I do:<br>
  <code>sudo apt-get update</code><br>
  <code>sudo apt-get install flex</code><br>
  <code>sudo apt autoremove</code> (because after installation the command line recommended me to do this) </p>
  
  <p>Now it seems to me that flex has been installed, I try <code>cmake -G "Unix Makefiles" ..</code> again...same error. I close the command line, start it up again, try it again - same error...<br>
  So now I'm slowly gettin' pissed and turn to Stack Overflow for help :D<br>
  What am I doing wrong???  </p>
  
  <p>And because I saw someone asking this in the other question's comment, here's the output of <code>flex</code>:</p>
  
  <pre><code>test@test-VirtualBox:~/doxygen/build$ flex
  The program 'flex' can be found in the following packages:
   * flex
   * flex-old
  Try: sudo apt install &lt;selected package&gt;
  </code></pre>