Resolves: rhbz#1092589 Thoroughly check whether JRE is still present

...not only on Mac OS X.  Was able to reproduce this on Fedora 20 where current
JRE was /usr/lib/jvm/java-1.8.0-openjdk-1.8.0.5.x86_64 but for whatever reason
there was also a left-behind /usr/lib/jvm/java-1.8.0-openjdk-1.8.0.0.x86_64 tree
(containing just a handful of sub-dirs, but no real content) that was still
recorded in my ~/.config/libreoffice/4/user/config/javasettings_Linux_X86_64.xml

Change-Id: Ie477c5a506a430f6c29525f6c558dbc18bbf1c48