MDEV-25777: JAVA_INCLUDE_PATH and JAVA_INCLUDE_PATH2 not found with out of source configuration and Ninja generator

- As solution `PLUGIN_CONNECT=NO` use early check to disable plugin:
  Solution suggested by wlad@mariadb.com
- `JNI_FOUND` is a internal result variable and should be set with
cached library and header variables (like `JAVA_INCLUDE_PATH`) defined.
  * Note: wrapper cmake/FindJNI.cmake runs first time and cmake native Find<module> returns only cached variable, like `JAVA_INCLUDE_PATH`, results variable are not cached).

Reviewed by: serg@mariadb.com