[PATCH] Implement LWG 2686, hash<error_condition>

[Daniel Krügler]

The following is an *untested* patch suggestion, please verify.

Notes: My interpretation is that hash<error_condition> should be
defined outside of the _GLIBCXX_COMPATIBILITY_CXX0X block, please
double-check that course of action.

I noticed that the preexisting hash<error_code> did directly refer to
the private members of error_code albeit those have public access
functions. For consistency I mimicked that existing style when
implementing hash<error_condition>.

- Daniel
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