FindFFmpeg.cmake: manages REQUIRED statement

* If REQUIRED is specified for modules, raise error if a module is not
found.
FindFFmpeg.cmake: set FFMPEG_FOUND if FFmpeg is found on the system
CMake: clean find java package

* No need to specified the jdk version (it could be different on an
other computer).
* Use the JAVA_HOME environment variable to get the jdk (make sure your
env. is correctly set).