Allow JBOSS_HOME to point to a symlink.

The cmake script is currently checking if the path specified in JBOSS_HOME is a directory. For symbolic links this check fails (e.g. JBOSS_HOME=/opt/jboss-as where /opt/jboss-as is a link to /opt/jboss-as-7.1.1.Final).

To resolve this problem the script was changed to check if JBOSS_HOME is set or not and use the value if present as a default for HOTROD_JBOSS_HOME.