[boost] [System] Non-standard code in error_code_user_test

The code in lib/system/test/error_code_user_test.cpp attempts to default-initialize a const-qualified variable non-POD class type that does not have a user-declared default constructor, which is ill-formed per C++03 [dcl.init]p9 (C++0x [dcl.init]p6). The attached patch fixes the problem be introducing a user-declared default constructor for two of the types we're testing with. - Doug