[patch] Add std::void_t

[Jonathan Wakely]

Define std::void_t for -std=c++17 and -std=gnu++11 (like Ville's
uncaught_exceptions() patch I think this is useful to define as an
extension pre-C++17, and the feature-test macro makes it easy for
users to test for the presence/absence of it in different modes).

Tested ppc64le-linux, committed to trunk.
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