test defs: fix 'javac' requirement for older JDK versions

test defs: fix 'javac' requirement for older JDK versions

The Java compiler from JDK 1.5 (and presumably earlier versions)
cannot handle the `-version' option by itself; and while it does
print the version number, it then errors out with an usage error:
  $ javac -version
  javac 1.5.0_22
  javac: no source files
  Usage: javac <options> <source files>
  ...
Luckily, adding the `-help' option to the `javac' invocation
seems to fix this problem.

* tests/defs.in (javac): Pass also the `-help' option to the
`javac' program.  Add a comment explaining why it is needed.

Report from Ralf Wildenhues.