Hooray, success! It seems I made too many updates before.
This is what IS necessary:
I started fresh from the VirtualBox image (it's from 2017-04-19 19:03:25).
sudo sed -i -re 's/([a-z]{2}\.)?archive.ubuntu.com|security.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list
sudo apt-get update
sudo apt-get install libmpc-dev
sudo apt-get install gawk
git clone git://git.rockbox.org/rockbox
mkdir build
cd build/
Then compiling the buildtools works:
sudo ../tools/rockboxdev.sh
And compiling for the Rocker then works, too. The UISimulator is ok, too.
Thanks to all!