Hi Jens,
I added this line in /etc/apt/sources.list:
deb [trusted=yes] https://dl.bintray.com/tango-controls/debian stretch main
Then I did:
sudo apt update
sudo apt install libtango9lts-dev
This installed the include files under /usr/local/include and the 9.3.3 lib under /usr/local/lib on my Debian Stretch VM.
Hoping this helps.
Reynald