This fixes handle_contract_violation under versioned namespace mode.

Tested under Linux x64 and confirmed to also fix Darwin build.

libstdc++: [_GLIBCXX_INLINE_VERSION] Provide handle_contract_violation 
symbol

libstdc++-v3/ChangeLog:

         * src/experimental/contract.cc
         [_GLIBCXX_INLINE_VERSION](handle_contract_violation): Provide 
symbol
         without version namespace decoration for gcc.

François