Set and save MACOSX_DEPLOYMENT_TARGET during translation (2.7)

MACOSX_DEPLOYMENT_TARGET specifies which OS version to target. It is commonly set using an environment variable, so I added the ability to do so for the translation options. The default is unchanged, but the MacPorts port always sets this to the current OS, so this should allow me to remove some patches from it.

One benefit of specifying this in the translation options is that tests shouldn't be affected by it, ensuring maximum coverage.

The clock_gettime() function was added in macOS 10.12, but forcibly disabled in PyPy. Now that we can rely on checks actually detecting which OS to target, I adjusted the check to reject it when targeting 10.7, even though it isn't available in CPython 2.7 on macOS. The main motivation is that it provides a good canary check; if the function is present in products from the buildbot, this branch doesn't work.

This replaces !758 (closed).

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