Previously we only used it on GCC 6 to avoid deprecated declaration
warnings. Now we are proactive and use it whenever compiling as C++11
(or MSVC2010+).
It also moves the logic for deciding between unique_ptr and auto_ptr
into a single location in config.h.
This fixes some use cases that were previously broken, including:
* CXX=clang++ -std=c++11 -Werror=deprecated-declarations
* CXX=g++-6 -std=c++03 -Werror=deprecated-declarations
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