cmake: Minor fixes to handle some CMake specific behavior
cmake: Minor fixes to handle some CMake specific behavior

Now, all source files are ignored that are not part of the
language of the target. This is also what CMake does.
Additionally it is now supported to build source files that
are generated inside the build directory.

pexip/meson,mesonbuild/meson,pexip/meson,QuLogic/meson,mesonbuild/meson,jpakkane/meson,mesonbuild/meson,jpakkane/meson,QuLogic/meson,becm/meson,QuLogic/meson,becm/meson,jpakkane/meson,becm/meson,pexip/meson,pexip/meson,becm/meson,QuLogic/meson,jpakkane/meson,jpakkane/meson,pexip/meson,jpakkane/meson,becm/meson,jpakkane/meson,pexip/meson,pexip/meson,becm/meson,QuLogic/meson,pexip/meson,QuLogic/meson,jpakkane/meson,jpakkane/meson,becm/meson,QuLogic/meson,mesonbuild/meson,QuLogic/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,QuLogic/meson,becm/meson,becm/meson,mesonbuild/meson,becm/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson