cc -O2 -DPOSIX -D_POSIX_C_SOURCE=200809L -D_FILE_OFFSET_BITS=64 -DTEST_NARROW_WRITES -c

This will be used to compile .c files.

If you use GCC 12, it emits a warning for something which is correct code.
The warning message looks like an error:

    memtester.c:142:9: error: suggest parentheses around assignment used as
        truth value [-Werror=parentheses]

You can ignore the warning message; the code is correct.  If the warning message
bothers you, you can add `-Wno-parentheses` to the options above.
