Fix generic build on Max OS X 10.7 Lion

    Recover variable build_mac_version which is removed on
    commit 644dc164 and added on commit 9ce06f11.

    Without this, ranlib libSDL.a is executed on Lion which
    causes build fail of emulator-arm.

Change-Id: I06144a288921f8f968ef457999398c1b9152d4aa