-> install build dependencies:
$ tce-load -wi compiletc cmake ffmpeg4-dev libpng-dev

-> per TC wiki, for maximum compatibility:
$ export CFLAGS="-mtune=generic -Os -pipe"
$ export CXXFLAGS="-mtune=generic -Os -pipe"
$ export LDFLAGS="-Wl,-O1"

-> finally:
$ cd ffmpegthumbnailer-2.2.2
$ cmake -DCMAKE_INSTALL_PREFIX=/usr/local -DCMAKE_BUILD_TYPE=Release -DENABLE_GIO=ON -DENABLE_THUMBNAILER=ON .
$ find . -type f | xargs sed -i 's/lib64/lib/'
$ make
$ mkdir /opt/dest; sudo make DESTDIR=/opt/dest install