MemoryBarrier() is used in the implementation, so it should be checked.
This in effect disables the faster atomic ops on msvc 7.1 and older, even if
only the MemoryBarrier() call is missing. But it ensures for the fist cut
that it will build everywhere. If somebody cares for msvc 7.1 enough and has
one for testing installed, he might provide the apropriate defines to guard
that MemoryBarrier() call.
I tested that msvc8 32/64bit still passes the configure tests and compiles.
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