Make libbluray build also on architectures without Java
- exclude the
default-jdk
build dependency onhppa
and Hurd, as there is no OpenJDK on these architectures - export JDK_HOME only in case the default JDK is installed: in case it is not available, the local jni headers will be used instead
- create a symlink to use the Linux versions of the local jni headers for Linux also on Hurd: the local jni headers for Linux and BSD are the same, and they should be fine also for Hurd
- drop the warning about possible FTBFS in case jni.h is not found, as it is most likely not true