[#2451] Add support for OS X 10.8.

Problem
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We want to support OSX ... at least so that developers can work on the project from OSX.

Changes
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For now, add explicit support for 10.8 as this is the only system I have tested...
it might also work for 10.7 or 10.9 but we don't have templates/slaves to test.

How to test
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reviewers: @dumol 

check that changes are ok. Thanks!