I want to mention that the question in the topic name is now not actual for me concerning building dependency for gEDA-gaf 1.10.2. On geda-project.org site the sources that are proposed to download require guile >= 2.2. But authors in the git version rolled back the requirements for guile version back to >= 2.0.11, so I don't need another guile version to build gEDA-gaf 1.10.2.
Still the question remains why the latest GNU C compiler don't build GNU extension language properly :-) If it may help I can try some digging but now I don't know in what direction. Building new guile version with gdb support and running the failing file under the debugger? Not a piece of cake, I guess :-)