Need new build property for minimum macOS version

[jlholt]

I'll add minOS=10.7 to the build.properties file in a pull request. Do you have preferred nomenclature for branching from master (that is, for naming the associated branch for the pull request)?

[sreilly]

Great question, and thanks in advance for the PR. Ideally it would be something like feature/<issuenumber>-<issuename> but anything is fine, really.

[sreilly]

for minimum build versions that can be done with a general property, but a specific property would also be nice