InterlockedExchangeAdd64 implementation for XP

Prevent OpenImageIO.dll to fail when dynamically loaded on XP.
Compilation for the XP target must be explicitely selected.
Fix strutil.cpp for WIN32

Prevent collision with max() macro
Back out thread.h changes from a while back -- they had unintentional
performance degredation.  It was incorrect to force only one of
USE_GCC_ATOMICS and USE_TBB to be true; there were some places (such
as atomic_compare_and_exchange where the gcc intrinsics were used if
available, even if TBB was being used, and apparently the gcc intrinsics
were much faster.
CHANGES and bump version for 1.1.7