Compiling external libraries takes a long time by now. Currently this happens very early in the compilation phase, before the other thorns are compiled.
It would be better if the compilation happened at the same time as that of other thorns.
This will entail splitting the configuration ie. detecting whether to build the included source or use a system wide copy from the compilation, most likely either using Cactus makefile variables or temporary files to transfer the decisions from the configuration stage to the build stage.