Allow using JDK 7 bootclasspath when compiling JGit using Java 8

Allow using JDK 7 bootclasspath when compiling JGit using Java 8

When compiling jgit using Java 8 set system property JDK_HOME to
JAVA_HOME path of JDK7 installation to compile against JDK 7 class
libraries. Otherwise jgit may hit runtime exceptions when running on
Java 7 (e.g. return type of ConcurrentHashMap.keySet() in JDK 8 class
library doesn't exist in JDK 7).

Example:
$ mvn clean install -DJDK_HOME=/.../jdk1.7.0_80.jdk/Contents/Home

Bug: 496262
Change-Id: Ib6abe5e544e0492e08b342e1a34b182caf25f94f
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>