Fix test for availability of emplace_back.
    
    Testing for the GCC version 5 or later isn't right, since C++ 11 support
    wasn't enabled by default until later.  This patch tests the C++ standard
    support directly instead of inferring it from the GCC version.
    
    gold/
    	* incremental.cc (Sized_incremental_binary::setup_readers): Use
    	emplace_back for C++ 11 or later.