configure: Add -D__USE_MINGW_ANSI_STDIO=1 to CPPFLAGS on MinGW32

This enables a more C99-compliant implementation of printf et al.

configure

@@ -3422,7 +3422,7 @@ probe_libc(){
         check_${pfx}cpp_condition _mingw.h "__MINGW32_MAJOR_VERSION > 3 || \
             (__MINGW32_MAJOR_VERSION == 3 && __MINGW32_MINOR_VERSION >= 15)" ||
             die "ERROR: MinGW32 runtime version must be >= 3.15."
-        add_${pfx}cppflags -U__STRICT_ANSI__
+        add_${pfx}cppflags -U__STRICT_ANSI__ -D__USE_MINGW_ANSI_STDIO=1
     elif check_${pfx}cpp_condition crtversion.h "defined _VC_CRT_MAJOR_VERSION"; then
         eval ${pfx}libc_type=msvcrt
         # The MSVC 2010 headers (Win 7.0 SDK) set _WIN32_WINNT to