Makefile requires c89 but test.c doesn't comply with that standard

[SeanHeelan]

$ make
cc -std=c89 -pedantic -Wall -Wextra -Werror -ggdb3 -O0 -c test.c -o test.o
test.c: In function ‘test_fill_with_deleted_items’:
test.c:201:5: error: ‘for’ loop initial declarations are only allowed in C99 or C11 mode
     for (int i = 0; i < 8; ++i)
     ^~~
test.c:201:5: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code
make: *** [Makefile:7: test.o] Error 1

$ cc --version
cc (GCC) 8.2.1 20181011 (Red Hat 8.2.1-4)
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