It's from revision 18541, and I just bumped into it on Debian. It looks like that function is present in guile 1.8, but not 1.6. Right now, one can't build GnuCash against Debian's guile 1.8 packages yet, because no guile-1.8-slib is provided. There's a Debian bug about this, #441110, but it looks like it's slipped through the cracks or something.