If the system has a system copy of the spidermonkey engine installed, it is ignored. This is reported under bugs.
It might be nice to slightly decouple FreeWRL from its own internal SpiderMonkey core and allow allow for 3 possibilities during build:
1) Build against a SpiderMonkey distribution included with FreeWRL.
2) Build against SpiderMonkey libraries (libjs/libjscript) installed on the system.
3) Build against a Rhino distribution included with FreeWRL.
Personally, I see the potential to have Rhino allow for more platform-independence and cleaner builds.
Plus, if there is already a SpiderMonkey -or- Rhino already installed, why not test them and use them if they pass?
Just some thoughts as I hack my way through trying to build a complete FreeWRL on this FC1/NVidia box.