Need help changing a makefile to static linking

Hi all,

I'm having a hard time compiling a plugin (softhddevice) for the video disk recorder software (vdr). Unfortunately the last official version of the plugin was published back in 2013 and even worse it has to be compiled against ffmpeg. The last working version of ffmpeg was 2.8.x, patches should make it work with 3.5.x (if you can find them). It's definitely not working with ffmpeg 4.x
So my thought was to forget about all the trouble with different versions and compile the plugin against a static built and working version of ffmpeg.
That was the quickly found plan, but the implementation is more than difficult if you are not used to deal with makefiles.
I tried to copy parts of the makefile(s) of tvheadend (which btw does exactly what I have in mind) that simply adds the .a files to the linking command. But without success. I know that there's the possibility to tell pck-config to use static libraries, but I have no glue where exactly in the makefile I have to insert this command and how I can force pkg-config to search within the ffmpeg directory.
I hope someone can push me in the right direction.
Thanks in advance
Alex