Update doxygen to version 1.8.20 / rev 81 via SR 841450
    
    https://build.opensuse.org/request/show/841450
    by user pgajdos + dimstar_suse
    - Use gcc9 for building on SLE_15 based distributions to workaround
      gcc7 bug PR85180.
    - Adjust building with libclang support for doxygen-1.8.20.
      - doxygen-llvm-libs.patch
      - doxygen-libclang-cpp.patch
      + doxygen-no-libclang-cpp.patch
    
    - Use gcc9 for building on SLE_15 based distributions to workaround
      gcc7 bug PR85180.
      - doxygen-llvm-libs.patch
      - doxygen-libclang-cpp.patch
      + doxygen-no-libclang-cpp.patch