Added support for CMake Extra Generators.
This adds support for CMake Extra generators and allows e.g. importing the build directories as regular projects in Eclipse IDE (and others).
Extra Generators are fully compatible to one of the traditional generators (Unix Makefiles or Ninja) but will generate additional files, usually to allow opening the project in an IDE like Eclipse or CodeBlocks. See: https://cmake.org/cmake/help/latest/manual/cmake-generators.7.html#extra-generators
This commit simply adds these extra generators as aliases for Unix Makefiles/Ninja. It's probably better to parse the generator name and extract the "base generator" from it programmatically (in case CMake adds more extra generators in the future), but I have zero knowledge of Perl so I just did it the primitive way.