MacOSX-Framework: Make script work in Xcode 4.0 and later

Apple made a number of changes to Xcode 4. The SDKs were moved, the entire
Developer folder was moved, and PowerPC support was removed. The script
will now adapt to those changes and should be future-proofed against
additional changes in case Apple moves the Developer folder ever again.
Also, the minimum OS X version compiler option was removed, so that the
framework can be built against the latest SDK but still run in older cats.