Fixed: Remove explicit use of Mac OS X 10.5 SDK, replace with "latest OS X"

The fontinfo and imagesize Xcode projects still explicitly refer to using the Mac OS X 10.5 SDK, while all other Xcode projects simply refer to the latest Mac OS X SDK, with no explicit OS version. This change modifies these two projects to mirror other Xcode projects in the project.

Fixes #1957
Merge pull request #1958 from GlennAustin/gla/NoOSX10.5

Fixed: Remove explicit use of Mac OS X 10.5 SDK, replace with "latest OS X"