Select javac source target from make variables

Author: Baraa-Hasheesh

Makefile

@@ -208,7 +208,7 @@ build/$(CONVERTER_JAR): $(CONVERTER_SOURCES) $(RESOURCES)
 	$(RM) -r build/converter
 
 %.class: %.java
-	$(JAVAC) -source 7 -target 7 -Xlint:-options -g:none $^
+	$(JAVAC) -source $(JAVA_TARGET) -target $(JAVA_TARGET) -Xlint:-options -g:none $^
 
 build/test/cpptests: $(CPP_TEST_SOURCES) $(CPP_TEST_HEADER) $(SOURCES) $(HEADERS) $(RESOURCES) $(JAVA_HELPER_CLASSES)
 	mkdir -p build/test