Set and save MACOSX_DEPLOYMENT_TARGET during translation (3.7)

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.

This is the same changes as !758 (closed), but targeting the py3.7 branch, as the logic in distutils differs.

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