I've installed xCode 4 last night and tried to compile a software which should support 10.5 and above OSs.
It gave the error message "Base SDK missing". I found that 10.5 base SDK is no longer in the menu.
Is there any way to get that base SDK? Is there any get around for this problem?
Thanks in advance.
Dropping the 10.5 SDK is also a problem for software which does not use Xcode for the build system, but expects (perhaps foolishly) for specific SDKs to be in place.
To add the SDK back, I re-installed the MacOSX10.5.pkg from the Xcode 3.2.6 download. It is in a hidden folder named "Packages" on the disk image. After mounting the .dmg file, you can open it from the command line with open /Volumes/Xcode\ and\ iOS\ SDK/Packages/. When installing the package, choose change install location and option-click on the drive you want to install to, so that you can specify a folder. The target folder should be /Developer or the base of your Xcode 4 install if you have put it in a non-standard location.
You can copy it from another Xcode installation into the /YourDeveloperFolder/SDKs/... but Xcode 4 still will not build Universal (Intel/PPC) binaries, essentially making it all but pointless to use the 10.5 SDK (except for "this method doesn't exist yet" style warnings). The official word from Apple (via their devforums): If you need 10.5/PPC support, stick with Xcode 3.
来源：https://stackoverflow.com/questions/5378518/how-to-add-base-sdk-for-10-5-in-xcode-4