@twobob: Usually, a quick round of checking the system's headers + google will get you the answer.
If it's a project using autotool, there's generally a configure-time check responsible for checking the system's libc that may be overridden manually. But you may also need/want to hack around the sources/headers of the project directly (see what I did for coreutils for example) to dumb it down.
grep -r & readelf are your friends
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The scanf case might be one of the trickier, if I remember correctly, you'll have to either build with gcc set to a particular -std that doesn't handle C99, or build with _GNU_SOURCE defined. That might not be viable workarounds for every projects out there...