The program doesn't work properly with xscreensaver. The following source file needs to be added (it seems to be missing by default): http://www.epcc.ed.ac.uk/~spb/xmountains/current_source/vroot.h and then the make command needs to be amended: make EXTRA_DEFINES=-DVROOT
It's fixed in xmountains-2.5-4. you will find it in rawhide soon