Add 'native' support for using Ninja with CMake for KDE projects

With this change the user may now configure their preferred CMake generator in kdesrc-buildrc.

This change introduces two new options to kdesrc-buildrc:

• cmake-generator as high-level alternative to passing -G explicitly in cmake-options
• ninja-options as Ninja analog to make-options

Additionally, cmake-options is now scanned for the pattern -G followed by a generator (e.g. -G "Unix Makefiles" or -GNinja). If set it will take precedence over cmake-generator, if neither is set the Unix Makefiles used as default. The chosen generator is explicitly passed to cmake during configure.