The printf ' format specifier is not Standard C. It's in neither the C99 Standard nor the new C11 Standard. So it's not actually valid, and it's a coincidence if it happens to work with your Standard Library.
Consider that the compiler generating that warning knows only Standard C, and in fact you could be pairing it with any C library, including those that are strictly conforming and don't support the ' extension.
Consider that the compiler generating that warning knows only Standard C, and in fact you could be pairing it with any C library, including those that are strictly conforming and don't support the ' extension.