Thank you so far.
But I have 20.04 working with the Intel drivers. So system ffmpeg is working with vaapi.
some variables in the auto script build for ffmpeg seem not to be compatible with the drivers I am running.
As I mentioned, I can build the tvheadend ffmpeg version with the following variables.
$ cd /tmp/tvheadend/build.linux/ffmpeg/ffmpeg-4.1.5
$ FFMPEG_PREFIX=/tmp/tvheadend/build.linux/ffmpeg/build/ffmpeg PKG_CONFIG=/tmp/tvheadend/support/pkg-config.ffmpeg ./configure --prefix=/ffmpeg --enable-shared --enable-vaapi --enable-gpl --enable-libx264 --enable-libx265
there must be a way, either to let tvheadend use the system ffmpeg or to adjust the variables autobuild uses.