Delta updates code uses libxar, which is only available in 10.5+, and
Sparkle framework fails to load on 10.4 because of it. Instead of having
hard dependency on libxar, Sparkle now links with it weakly and checks
for its availability at runtime. This required bumping Xcode project
compatibility to 3.2 to get weak linking support.
With this change, delta updates are still not supported on 10.4, but
Sparkle runs there again and just ignores the deltas. This should be
good enough given 10.4's age and shrinking user base.