Unknown CMake command "doxygen_add_docs".

**Describe the bug**
```
CMake error when running the build.sh script. It errors out at the end with::
CMake Error at trantor/CMakeLists.txt:357 (doxygen_add_docs):
**Unknown CMake command "doxygen_add_docs".**

-- Configuring incomplete, errors occurred!
```

**To Reproduce**
Steps to reproduce the behavior:
1. git clone https drogon ; install dependencies
2. cd to /drogon/ folder
3. run ./build.sh
4. See error

**Expected behavior**
I expected it to either request to install dependencies or successfully build.

**Screenshots**
```
./build.sh
Deleted folder: ./build
Created building folder: ./build
Entering folder: ./build
Start building drogon ...
-- The C compiler identification is GNU 6.3.0
-- The CXX compiler identification is GNU 6.3.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- compiler: GNU
You have called ADD_LIBRARY for library drogon without any source files. This typically indicates a problem with your CMakeLists.txt file
-- Performing Test COMPILER_HAS_HIDDEN_VISIBILITY
-- Performing Test COMPILER_HAS_HIDDEN_VISIBILITY - Success
-- Performing Test COMPILER_HAS_HIDDEN_INLINE_VISIBILITY
-- Performing Test COMPILER_HAS_HIDDEN_INLINE_VISIBILITY - Success
-- Performing Test COMPILER_HAS_DEPRECATED_ATTR
-- Performing Test COMPILER_HAS_DEPRECATED_ATTR - Success
-- Looking for C++ include any
-- Looking for C++ include any - not found
-- Looking for C++ include string_view
-- Looking for C++ include string_view - not found
-- Looking for C++ include coroutine
-- Looking for C++ include coroutine - not found
You have called ADD_LIBRARY for library trantor without any source files. This typically indicates a problem with your CMakeLists.txt file
-- Could NOT find OpenSSL, try to set the path to OpenSSL root folder in the system variable OPENSSL_ROOT_DIR (missing: OPENSSL_LIBRARIES OPENSSL_INCLUDE_DIR)
-- Found PkgConfig: /usr/bin/pkg-config (found version "0.29")
-- Could NOT find Botan (missing: Botan_LIBRARIES Botan_INCLUDE_DIRS)
-- Trantor using SSL library: None
-- Found c-ares: /usr/include
-- c-ares found!
-- Looking for pthread.h
-- Looking for pthread.h - found
-- Looking for pthread_create
-- Looking for pthread_create - not found
-- Looking for pthread_create in pthreads
-- Looking for pthread_create in pthreads - not found
-- Looking for pthread_create in pthread
-- Looking for pthread_create in pthread - found
-- Found Threads: TRUE
-- Found Doxygen: /usr/bin/doxygen (found version "1.8.13")
**CMake Error at trantor/CMakeLists.txt:357 (doxygen_add_docs):
Unknown CMake command "doxygen_add_docs".**

-- Configuring incomplete, errors occurred!
See also "/var/www/drogon/build/CMakeFiles/CMakeOutput.log".
See also "/var/www/drogon/build/CMakeFiles/CMakeError.log".
```

**Desktop (please complete the following information):**
```
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```

**Additional context**
Seems similar to what happened here:
https://github.com/conan-io/cmake-conan/issues/330