Provide a compatibility wrapper for scandir in legacy-support

[fhgwright (Fred Wright)]

The signature for scandir() changed between the 10.7 and 10.8 SDKs, causing a pointer-type mismatch in some builds. This is usually just a warning, but gcc14 started treating this as an error by default, causing broken builds.

The fix should consist of creating a wrapper function to "launder" the argument type, and then optionally defining scandir as a macro expanding to the wrapper. This can't be done unconditionally, since that could break anything that assumes the default behavior.