GccToolChain: Make macroCache handling accessible

Make the macro Cache accessible to derived classes. This is done
so that we can populate that cache with the data the factory/config widget
already has and thus reducing the number of times we need to run the
compiler.

Change-Id: I5ea72a0688c765ae995db66dde3ed6f6672198b6
Reviewed-by: Daniel Teske <daniel.teske@digia.com>