I had, when building any output with synfig, this error message on the console:
synfig: warning: Unable to find module “mod_libavcodec” (mod_libavcodec.so: cannot open shared object file: No such file or directory)
After installing libavformat-dev on my system, the problem is gone. This applys to all debian based linux distributions like Ubuntu, Kubuntu, Xubuntu…
This is a known phenomenon on debian based distributions, because they only install the files needed to run a program, not to build it. So if something doesn’t work, check if a missing “-dev” package is the cause.
This is covered in the build instructions wiki page, a forum message wasn’t needed. Anyone building from source should know about -dev/-devel packages anyway.