Revert "Xcode.prefix: remove osx-gcc-installer hack"
Revert "Xcode.prefix: remove osx-gcc-installer hack"

This reverts commit d9f6616738e1af715162b804c9585d32522a0ef5.

This turns out to break Xcode version detection on Tiger; reverting
this until a better longterm solution is found.

ahihi/tigerbrew,ahihi/tigerbrew,ahihi/tigerbrew,ahihi/tigerbrew