Log Message:
Changes 1.51.0:
New Libraries
* Context: Context switching library, from Oliver Kowalke.
Updated Libraries
* Algorithm:
- Fixed is_sorted_until ; now matches the c++11 standard behavior on ranges with
  duplicate values.
- Added range support for the rest of the search algorithms.
- unhex now uses Boost.Exception to signal bad input.
* Asio:
- Fixed an incompatibility between ip::tcp::iostream and C++11.
- Decorated GCC attribute names with underscores to prevent interaction with
  user-defined macros.
- Added missing #include <cctype>, needed for some versions of MinGW.
- Changed to use gcc's atomic builtins on ARM CPUs, when available.
- Changed strand destruction to be a no-op, to allow strand objects to be
  destroyed after their associated io_service has been destroyed.
- Added support for some newer versions of glibc which provide the
  epoll_create1() function but always fail with ENOSYS.
- Changed the SSL implementation to throw an exception if SSL engine
  initialisation fails.
- Fixed another regression in buffered_write_stream.
- Implemented various minor performance improvements, primarily targeted at
  Linux x86 and x86-64 platforms.
* Config:
- Deprecated a whole bunch of macros that were c++11 specific, but not named to
  show that they were c++11 specific. Made new macros with better names, and
  paired the old macros with the new ones. Updated the documentation to list
  the deprecated macros.
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