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/i2psnark/java/bundle-messages.sh

@@ -15,6 +15,10 @@ TMPFILE=build/javafiles.txt
 export TZ=UTC
 RC=0
 
+if ! $(which javac]; then
+    export JAVAC=${JAVA_HOME}/../bin/javac
+fi
+
 if [ "$1" = "-p" ]
 then
 	POUPDATE=1