4.1.0

Many small bug fixes in this release. But there are some new features also.
There's now an lto feature to specify the use of LTO, and what kind. The
existing stdlib feature now has real values and corresponding options
for some toolsets. But most importantly there's new documentation for all
the features.

Thank to all the users that contributed to this release with these changes:

• Support for VS2019 for intel-vin 19.0. -- Edward Diener
• Fix compiler warnings about -std=gnu11 when building b2 on Cygwin. --
  Andrey Semashev
• Add example of creating multiple PCHs for individual headers.
  -- Rene Rivera
• Add QNX threading flags for GCC toolset. -- Aurelien Chartier
• Fix version option for IBM and Sun compilers when building b2 engine
  -- Juan Alday
• Rename strings.h to jam_strings.h in b2 engine to avoid clash with
  POSIX strings.h header. -- Andrey Semashev
• Add options for cxxstd feature for IBM compiler. -- Edward Diener
• Many fixes to intel-win toolset. -- Edwad Diener
• Add z15 instruction set for gcc based toolsets. -- Neale Ferguson
• Improve using MSVC from a Cygwin shell. -- Michael Haubenwallner
• Add LTO feature and corresponding support for gcc and clang toolsets.
  -- Dmitry Arkhipov
• Fix errors when a source doesn't have a type. -- Peter Dimov
• Add documentation for features. -- Dmitry Arkhipov
• Enhance stdlib feature, and corresponding documentation, for clang, gcc,
  and sun toolsets. -- Dmitry Arkhipov
• Install rule now makes explicit only the immediate targets it creates.
  -- Dmitry Arkhipov
• Add armasm (32 and 64) support for msvc toolset. -- Michał Janiszewski
• Fix errors with custom un-versioned gcc toolset specifications.
  -- Peter Dimov
• Allow arflags override in gcc toolset specifications. -- hyc
• Fix founds libs not making it to the clang-win link command line.
  -- Peter Dimov
• Updated intel-win toolset to support for Intel C++ 19.1.
  -- Edward Diener
• Detect difference between MIPS32 and MIPS64 for OS in b2 engine.
  -- YunQiang Su