It was a big pain in the butt to figure out how to independently build plugins and event handlers for Janus outside of the Janus repository build scripts, so here is a skeleton for building them using autotools.
The plugins/ directory contains a simple plugin, and the events/ directory contains a simple event handler. You can build them both with make at the top level, or build them individually in their respective directories.
The samples given are compatible with an old version of Janus (0.4.3). They will likely require modification to work correctly with newer versions. However, the build process and general plugin structure AFAIK has stayed the same.
./bootstrap
./configure --prefix=/opt/janus # or wherever your janus install lives
make
make install # installs into /opt/janus/lib/janus/plugins and /opt/janus/lib/janus/events