Yes, kinda advertized in HACKING -- but -Werror is harder to use because you need to check your flags VERY carefully not to produce any false positive. E.g. -Wwrite-strings have a few false positive due to imperfect API. But yeah, it's nice to stop on possible errors. FTR, we (devs) all (I believe) use this or stricter options (but maybe not with -Werror).
I think it's better to check compiler warning in 2 minutes than spending two days on debugging:) I prefer always enable all warnings in new projects. Clearly that Geany is big project and enabling all warnings may be quite painful. But it will certainly reduce number of errors - when you consider every line of code twice.
-Werror is a good practice for people who ignore compiler warnings:)
Some stupid warnings may be easily disabled (-Wno-unused-parameter).
-- Best regards Pavel Roschin aka RPG