Build: cmake polish, notably BUILD_TESTING option

[leamas]

Two minor cmake tweaks:

1. Add a guard against multiple inclusions which might happen when part of larger builds.
2. Make the BUILD_TESTING variable an option which can be set by user.

These changes should AFAICT be backwards compatible.

Exported from the opencpn project at github.com://opencpn/opencpn (Well, not really now, but soon).