legacy-support: functions properly only if os.version = deployment target

[kencu (Ken)]

while debugging some unexpected behavior recently, I realized the build system was setting the mmacosx-version-min to a value of 10.7 on a host os of 10.11. What this did was enable a bunch of replacement definitions in the legacy support headers that collided with definitions in the SDK.

This can be fixed by making the legacy support definitions exactly match the newer SDK's definitions (eg getattrlistat)' or by removing the deployment target change...

But it brought forth the idea that the exact way the legacysupport library is built depends on both the host os version but also the setting of the deployment target.

It may or may not be possible to build legacy support for an older system - that idea would have to be explored and tested -- but for now, at least, making sure the host os version matches the deployment target (and the sdk version?) is the only reliable way of using the legacy support library and PG...