MinGW: always use "-D__USE_MINGW_ANSI_STDIO"

This is unfortunately needed to make stdio work like OpenOCD expects -- matching
the ANSI-C standard, instead of MS-Windows.

I tested it in both MinGW-W64 on Vista 64 and MinGW-W32 on XP, and I don't
see any adverse effects to enabling it for all MinGW versions.

configure.in

@@ -482,6 +482,8 @@ case $host in
     fi
     parport_use_giveio=yes
     
+    CFLAGS="$CFLAGS -D__USE_MINGW_ANSI_STDIO"
+
     AC_DEFINE(IS_MINGW, 1, [1 if building for MinGW.])
     AC_DEFINE(IS_WIN32, 1, [1 if building for Win32.])
     AC_DEFINE(IS_DARWIN, 0, [0 if not building for Darwin.])