Impossible to build project without doxygen

[Tako-San]

Command line:

$ cmake .. -DCMAKE_BUILD_TYPE=Debug -DCMAKE_CXX_COMPILER=clang++-11 -DBUILD_DOC=OFF -DBUILD_TEST=ON

Error:

...
CMake Error at /usr/share/cmake-3.16/Modules/FindPackageHandleStandardArgs.cmake:146 (message):
  Could NOT find Doxygen (missing: DOXYGEN_EXECUTABLE doxygen)
Call Stack (most recent call first):
  /usr/share/cmake-3.16/Modules/FindPackageHandleStandardArgs.cmake:393 (_FPHSA_FAILURE_MESSAGE)
  /usr/share/cmake-3.16/Modules/FindDoxygen.cmake:633 (find_package_handle_standard_args)
  docs/CMakeLists.txt:2 (find_package)
...