From http://www.jwz.org/xscreensaver/xscreensaver-5.31.tar.gz

[xscreensaver] / OSX / xscreensaver.xcconfig

diff --git a/OSX/xscreensaver.xcconfig b/OSX/xscreensaver.xcconfig
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+// To create builds that work properly on MacOS 10.6, Xcode 5.0.2 must
+// be used, since that is the latest version that supports garbage collection.
+// If the Xcode version is 5.02 or earlier, this must be set to "-fobjc-gc".
+// If the Xcode version is newer than that, this must be blank (and the
+// builds created will not work on 10.6.)
+//
+OBJC_GC_CFLAGS=-fobjc-gc