[CMAKE] Add a status message to inform the user that ccache is used

Summary: If ccache is not found nothing is printed.

Test Plan:
With ccache installed:

cmake -GNinja .. | grep ccache

Should display something like:

• Using ccache: /usr/bin/ccache cmake -GNinja .. -DCCACHE=OFF | grep ccache

Should return nothing.

Reviewers: #bitcoin_abc, deadalnix, fpelliccioni

Reviewed By: #bitcoin_abc, fpelliccioni

Differential Revision: https://reviews.bitcoinabc.org/D4768