issue #1048: detect if java8 has already been installed

toolset/setup/linux/languages/java8.sh

@@ -1,13 +1,16 @@
 #!/bin/bash
 
+RETCODE=$(fw_exists java8.installed)
+[ ! "$RETCODE" == 0 ] || { return 0; }
+
 sudo add-apt-repository -y ppa:webupd8team/java
 sudo apt-get update
 echo oracle-java8-installer shared/accepted-oracle-license-v1-1 select true | sudo /usr/bin/debconf-set-selections
 sudo apt-get install -y oracle-java8-installer
 
-sudo ln -s /usr/lib/jvm/java-8-oracle /opt/java8
+sudo ln -sf /usr/lib/jvm/java-8-oracle /opt/java8
 
-# set Java 7 (OpenJDK) as the default, as previously.
+# set Java 7 (OpenJDK) as the default, as before.
 sudo update-alternatives --set java /usr/lib/jvm/java-7-openjdk-amd64/jre/bin/java
 sudo update-alternatives --set javac /usr/lib/jvm/java-7-openjdk-amd64/bin/javac
 
@@ -21,3 +24,5 @@ do
     sudo update-alternatives --set $n "$p"
   fi
 done
+
+touch $IROOT/java8.installed