[CMAKE] Add a target to print the current version number

Summary:
It can be difficult to search for the project version number, because of
the CMake syntax that makes it likely to grep the version from unrelated
packages.
This diff adds a simple print-version target that will display the
current version and allow for easy grepping.

Test Plan:

ninja print-version
ninja print-version | grep -E "[0-9]+\.[0-9]+\.[0-9]+"

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7359