Bug#935842: jh_buld: Make java source version configurable

[darkdragon]

Package: javahelper
Version: 0.72.8

In jh_build, line 15:

use constant DEFAULT_JAVA_RELEASE => '1.7';

Java 7 is chosen as a default. Building with debuild on latest Ubuntu
with OpenJDK11 gives the following warning:

warning: [options] bootstrap class path not set in conjunction with -source 7

Probably the best way would be to overwrite this with an environment
variable in debian/rules.