[ACE6] tao_idl Improvements

[iguessthislldo]

Backport of the appropriate features of #1357 to TAO 2.

• Added an IDL preprocessor macro for the IDL version, __TAO_IDL_IDL_VERSION.
• Added proper documentation for the __TAO_IDL IDL preprocessor macro.
• Fixed issue encountered in this OpenDDS PR where OpenDDS' idl2jni, which is a tao_idl-derived compiler, can't properly handle relative includes because tao_idl makes a temporary copy that is passed to the preprocessor. Unlike in #1357 guess is not the default for --preprocessor-input, but copy is, which is the current behavior.

[jwillemsen]

See my comments of #1357, why not set the preprocessor handling through config*.h files instead of guessing it in tao_idl.