OSX: Use the oldest SDK, but always set our target to 10.8

Build binaries usable on older machines even if older SDKs are not
installed. Older SDKs can no longer be installed on newer Xcode
versions.

Port of https://android-review.googlesource.com/207242

Change-Id: If66f2eef7593bce9fb858fc7b71369f55dd362e4
1 file changed