error: conflicting types for built-in function ‘tolower’ [-Werror]

I have a small RTOS which does not use glibc and I have written own functions (e.g. tolower) in string.c

When compiling I am getting the error:

common/string.c:11:6: error: conflicting types for built-in function ‘tolower’ [-Werror]

Is there a CFLAGS to fix this?

Update Answer: use -fno-builtin