minor Rakefile issue w.r.t. tools.jar

[Paul R Brown]

All --
There's a dependency on tools.jar in the current Rakefile, but  
tools.jar doesn't really exist.  It's a figment of SUN's JDK  
structure, and it's not present in Apple, IBM, or potentially other  
JDKs.  As is, Ode builds via rake  on MacOS without issue if line 375  
is commented out.

Implementing JDK detection magic in Ode's build file doesn't make  
sense, so this is more of an enhancement suggestion for buildr.  For  
the moment, wrapping it with if !PLATFORM.index('darwin') is enough  
to keep me happy.

Any objections?

-- Paul