Use a CMake-specific module-configured key for KDE builds.

Previous to this the GenericBuild fallback of 'Makefile' was used to
test if a configure step had been run, but this only works for CMake
when the default generator is being used.

The 'ninja' generator is available in the latest CMake and seems to be
popular. Instead of hard-coding support for ninja we just try to support
something CMake-specific. Those who wish to use ninja should be able to
use cmake-options to add the appropriate -G flag, and
custom-build-command to specify ninja.