JAVA_HOME... why?

[Jordi Gutiérrez Hermoso]

Can someone please explain to me again why we need JAVA_HOME? Why is
jni.h special and can't be installed and used like any other system
header? I can understand setting it if your Java installation is in a
non-default location, but it's really strange that the current trend
is to be telling anyone who builds Octave that they have to make sure
JAVA_HOME is set correctly; moreso for configure to be complaining
about it when it's not. The configure script doesn't complain if I
don't have QT_HOME or FFTW_HOME set correctly; why is Java special? I
am able to build Octave with Java support without JAVA_HOME. Why can't
anyone else?

I'm not saying this is a stupid thing; I just want to understand the problem.

- Jordi G. H.