Builing spidermonkey is not very well supported. I made some assumptions about which files to put into the packages. Of course, those assumptions may be wrong (but using them I got mediatomb compiled).

[sbox-ARM] ~
$ hfp get libjs

[sbox-ARM] ~
$ cd src

[sbox-ARM] ~/src
$ mv js/src libjs-1.7.0

[sbox-ARM] ~/src
$ cd libjs-1.7.0

[sbox-ARM] ~/src/libjs-1.7.0
$ make -f Makefile.ref

[sbox-ARM] ~/src/libjs-1.7.0
$ hfp create-skeleton

[sbox-ARM] ~/src/libjs-1.7.0
$ hfp copy Linux_All_DBG.OBJ/libjs.so usr/lib

[sbox-ARM] ~/src/libjs-1.7.0
$ hfp copy Linux_All_DBG.OBJ/*.h usr/include

[sbox-ARM] ~/src/libjs-1.7.0
$ hfp copy js*.h usr/include

[sbox-ARM] ~/src/libjs-1.7.0
$ hfp copy js*.tbl usr/include

[sbox-ARM] ~/src/libjs-1.7.0
$ hfp copy Linux_All_DBG.OBJ/js{,cpucfg,kwgen} usr/bin

[sbox-ARM] ~/src/libjs-1.7.0
$ hfp create-package -e