Out-of-source build touches files in source dir

mysql_version.h and my_config.h are generated files created from .in templates, but are created inside CMAKE_SOURCE_DIR instead of CMAKE_CURRENT_BINARY_DIR

so building out-of-source is not possible if the user doing the build doesn't have write access to the source dir, and when having multiple out-of-source builddirs with e.g. different CMAKE_INSTALL_PREFIX builds may overwrite each others changes to my_config.h

I guess placing the generated files in the source dir, not build dir, was intentional for cpack to be able to pick them up, but IMHO this is still something that needs to be fixed / handled in a different way

(... insert usual rant about automake "make dist" being more flexible about defining such extra dist files and "make distcheck" checking for exactly this kind of problem, amongst other things, here ...)