Enable support for Ninja build generator

If you have CMake 2.8.9 or later (you can get it from the Ubuntu SDK PPA if on precise), and you have Ninja build 1.0 (you can use the Quantal one as a backport on Precise), then you can enable Ninja builds (which is faster than Makefiles) using the `-GNinja` switch.

This changeset exposes the same functionality to the Autotools-compatibility layer (meaning that you can specify --enable-ninja to dunecontrol!)