Update clang to 12 and gcc to 8 and qt5.11.1 on Ubuntu xenial 16.04

update-gcc-clang-12-ubuntu-16.04-xenial.sh

if [ ! -f /usr/bin/gcc-8 ]; then
    sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
    sudo apt-get update
    sudo apt-get install -y gcc-8 g++-8

    sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-8 1000
    sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-8 1000
fi

if [ ! -f /usr/bin/clang-12 ]; then
    wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
    sudo apt-add-repository -y "deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-12 main"
    sudo apt-get update
    sudo apt-get install -y clang-12

    sudo update-alternatives --install /usr/bin/clang   clang   /usr/bin/clang-12    1000
    sudo update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-12  1000
fi


sudo add-apt-repository ppa:beineri/opt-qt-5.11.1-xenial
sudo apt update
sudo apt install qt511-meta-full

mkdir ~/.config/qtchooser
echo "/opt/qt511/bin" > ~/.config/qtchooser/default.conf
echo "/opt/qt511/lib" >> ~/.config/qtchooser/default.conf