OpenBSD compilation fix: set JAVAC in bundle targets if javac not found and require.gettext is set to true

This solves the following problem which was found on OpenBSD:

bundle:
     [exec] Generating net.i2p.desktopgui.messages_ar ResourceBundle...
     [exec] ERROR - msgfmt failed on locale/messages_ar.po, not updating translations
     [exec] msgfmt: Java compiler not found, try installing gcj or set $JAVAC
     [exec] msgfmt: compilation of Java class failed, please try --verbose or set $JAVAC
     [exec] 9 translated messages.


In OpenBSD 5.4, neither java nor javac are in the PATH.

apps/desktopgui/build.xml

@@ -44,6 +44,7 @@
 
 	<target name="bundle" unless="no.bundle">
         <exec executable="sh" osfamily="unix" failifexecutionfails="true" failonerror="${require.gettext}" >
+            <env key="JAVA_HOME" value="${java.home}" />
             <arg value="./bundle-messages.sh" />
         </exec>
         <exec executable="sh" osfamily="mac" failifexecutionfails="true" failonerror="${require.gettext}" >