Windows: Disable MinGW-w64's stdio functions in size-optimized builds

Windows: Disable MinGW-w64's stdio functions in size-optimized builds

This only affects builds with UCRT. With legacy MSVCRT, the replacement
functions are always enabled.

Omitting the MinGW-w64 replacements saves over 20 KiB per executable.
The downside is that --enable-small or XZ_SMALL=ON disables thousand
separator support in xz messages. If someone is OK with the slower
speed of slightly smaller builds, lack of thousand separators won't
matter.

Don't override __USE_MINGW_ANSI_STDIO if it is already defined (via
CPPFLAGS or such method).