murmur_ice: use `slice2cpp --version` to guess the slice include path for slice2cpp.
murmur_ice: use `slice2cpp --version` to guess the slice include path for slice2cpp.

Also, while we're here, remove the old list of hard-coded Ice versions
from slice2cpp's arguments.

Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble