It doesn't even need to see __builtin_strlen (try modify your string.h or use musl, which has neither the attribute nor __builtin_strlen). It just needs to see

It doesn't even need to see __builtin_strlen (try modify your string.h or use musl, which has neither the attribute nor __builtin_strlen). It just needs to see strlen in a hosted environment (with the right header included?). If you want gcc to treat it otherwise, you also need -fno-builtin-strlen.