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This downloads OS X 10.10 SDK, installs it and uses for compilation.

According to https://www.gosquared.com/global/mac/el-capitan/, current
market share among OS X versions is:
* 10.11 — 41.53%
* 10.10 — 38.65%
* 10.9  — 14.72%
* 10.8  — 5.60%
* 10.7  — 5.62%

I tried adding 10.9 SDK, but it fails to compile. With this change, we
should cover around 80% of OS X users.