CMake: MakeStep consistently check that we have a buildconfiguration

Technically the user can only get into a no buildconfiguration state
with broken settings. But we should either check everywhere or nowhere.

Change-Id: Ibdc831b66e6cf141c8d25bafed7e568e3cff2ab1
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
CMakeProject: Fix useNinja handling in MakeStep

m_useNinja is now only used to store the value at init() time, thus
it's only use is to know while running, whether ninja is in use.

Otherwise whether ninja is used or not, is stored in the
buildconfiguration.

Change-Id: Icda933807ac2f8e5fad32fd124a37bb694bf6e97
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>