In a post-9.5 change, GetConfigOptionByName() has been augmented with a true/false option... extern char *GetConfigOptionByName(const char *name, const char **varname, bool missing_ok); The call done in src/backend/slony1-funcs.c needs to offer the new parameter.
Patch in HEAD... http://git.postgresql.org/gitweb/?p=slony1-engine.git;a=commit;h=104b74588334ff85fc8540c37d68256446424ff5 It is likely desirable to add this to some back branches too.