Doxygen - Could NOT find FLEX (missing: FLEX_EXECUTABLE)

Question

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:
I'm trying to see which of my C++ methods are called by others so I found Doxygen after googling.
On their page the installation seems pretty straightforward:

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:

git clone https://github.com/doxygen/doxygen.git
cd doxygen
mkdir build
cd build
cmake -G "Unix Makefiles" ..
make

Until cmake -G "Unix Makefiles" .. everything goes well, then on that command following error occurs:

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) 

I thought: No big deal, why should anything work out of the box...seems I have to install flex.
So I do:
sudo apt-get update
sudo apt-get install flex
sudo apt autoremove (because after installation the command line recommended me to do this)

Now it seems to me that flex has been installed, I try cmake -G "Unix Makefiles" .. again...same error. I close the command line, start it up again, try it again - same error...
So now I'm slowly gettin' pissed and turn to Stack Overflow for help :D
What am I doing wrong???

And because I saw someone asking this in the other question's comment, here's the output of flex:

test@test-VirtualBox:~/doxygen/build$ flex The program 'flex' can be found in the following packages:  * flex  * flex-old Try: sudo apt install <selected package> 

Answer 1

It seems like apt autoremove really removed the package I just installed in the previous step.
So what worked for me was:

git clone https://github.com/doxygen/doxygen.git cd doxygen mkdir build cd build 

These are new:

---

sudo apt-get install flex sudo apt-get install bison 

---

cmake -G "Unix Makefiles" .. make 

...but of course the horror wouldn't end there, see my next question :D