https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104208
--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> --- I think the glibc problem is in a compiler defaulting to -mabi=ieeelongdouble they want to use -mabi=ibmlongdouble -mlong-double-64 (because -mabi=ibmlongdouble is added into CFLAGS everywhere early, and -mlong-double-64 is needed for a few TUs). That wouldn't be diagnosed in the #c4 case, -mabi=ibmlongdouble is used while -mlong-double-128 is in effect, so valid case. -mlong-double-64 -mabi=ibmlongdouble would be diagnosed though (-mabi=*double while -mlong-double-64 in effect).
