Make use of ccache optional (was ACTS-446)

Original author Stewart Martin-Haugh @smh

Hi,

While building ACTS I found that ccache was using a lot of space in my home directory. I also know that ATLAS builds currently avoid ccache, so I thought it would be good to make its use optional. MR incoming.

Edited Mar 27, 2019 by Moritz Kiehn

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