CMake: add Ninja support

The cmake wizard proposes also ninja if ninja support is available

Ninja must be in PATH, but it is only called once, so it doesn't hurt.

Task-number: QTCREATORBUG-7720
Reviewed-by: Daniel Teske <daniel.teske@digia.com>

Change-Id: If3c9c7ae55e6990fa16b031fc2998a8d8d9ed17a
Reviewed-by: Peter Kümmel <syntheticpp@gmx.net>
Reviewed-by: Yuchen Deng <loaden@gmail.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>