If you download the prebuilt win32 spidermonkey binaries for windows you get 4 libs, plus headers
js32r.lib
js32d.lib
js64r.lib
js64d.lib
For a window build scons should check to see what the envirorment is configured to build (32/64 bit and release or debug) and look for one of those after looking for the standard names for js.lib.