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>