MinGW: PRINTF_FORMAT_ARG workaround (#1390)

MinGW: PRINTF_FORMAT_ARG workaround (#1390)

MinGW produces warnings about not supporting the printf '%zu' format
code when the 'printf' format archetype is used to validate code.

Checking against the 'gnu_printf' format archetype avoids this bad
warning. We accept the lower rate of error detection since other OS
builds verify against the 'printf' format archetype.

We also removed undocumented, inconsistent, and presumably unused
support for providing custom PRINTF_FORMAT_ARG macros during Squid
build. This removal simplifies code.