[Libreoffice] Java build problem identified?

Dunno how to fix it, but I think I've found out what's wrong with my
build ... the following is output from autogen

checking whether to build with Java support... yes
checking for java... /usr/bin/java
checking the installed JDK... checked (JDK 1.6.0_20)
checking for target java bytecode version... 1.6
checking for javac... /usr/bin/javac
checking for javadoc... /usr/bin/javadoc
configure: WARNING: JAVA_HOME was not explicitly informed with
--with-jdk-home. the configure script
configure: WARNING: attempted to find JAVA_HOME automatically, but
apparently it failed
configure: WARNING: in case JAVA_HOME is incorrectly set, some projects
with not be built correctly

so it's clearly found the jdk, but it can't identify where it is. It's a
standard gentoo install and presumably had no problem finding it before
on the old build system. I've installed virtual/jdk-1.6.0. I'm guessing
icedtea is a jdk? Which is installed in /opt/icedtea6-bin-1.9.1. So the
build system ought to pick it up ...

However, I've now installed the Oracle jdk and pointed jdk-home at that,
and it seems (touch wood) to be building okay...

Cheers,
Wol