g++ 4.9.2 added support of __has_include() but, unlike clang, refuses to
compile the <type_traits> header that is detected as existing now in C++98
mode, so the build was broken with it when not using configure (i.e. under
MSW).
Fix this by, first, testing for C++11 compilers separately (which seems like a
good idea anyhow as it will allow using these headers with other compilers)
and, second, not trusting g++ __has_include() for C++11 headers in C++98 mode.
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