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              Value          Meaning

               -compat=4      Compile for compatibility with C++
                              4.0.1, C++ 4.1, and C++ 4.2 (Sets
                              __cplusplus macro to 1.)

               -compat=5      Compile with full C++ 5.0 features.
                              (Sets __cplusplus macro to
                              199711L.)

This suggests that you could check whether __cplusplus is one rather than 
whether __SUNPRO_CC is less than 0x500, and things would work.  I believe 
that this is Sun's recommended strategy.  

What do you think?

David