When compiling my FreeRTOS, my functions in string.c conflict with the built-in function 'tolower'

makefile
all: my_rtos

my_rtos: main.o string.o
    gcc -o my_rtos main.o string.o
    
main.o: main.c
    gcc -c main.c

string.o: string.c
    gcc -c -Dmy_tolower=custom_tolower string.c