Automatic PG_PRINTF_ATTRIBUTE

pg_config_manual.h has been choosing gnu_printf as the PG_PRINTF_ATTRIBUTE for
every MinGW build.  That invites a torrent of warnings on pre-gcc-4.4 MinGW
compilers, including the compiler on buildfarm member narwhal.  I'm
increasingly using an affected compiler, because it builds twice as quickly as
today's gcc.  Let's have "configure" detect whether gcc supports gnu_printf
before using it.  I gather plain "printf" aliases ms_printf on Windows and
gnu_printf elsewhere.  Therefore, while the new "configure" test applies to
all platforms, non-Windows platforms are disinterested in the outcome today.
Suppose gcc introduces aix_printf and has plain "printf" alias it on AIX.
PostgreSQL will continue to replace platform printf implementations that
depart from our format processing expectations, and our own elog.c code
processes errmsg() formats.  Therefore, gnu_printf would remain the better
global choice even if new archetypes become available.