4.7.0

Many, many fixes and internal cleanups in this release. But also adding
auto-detection and bootstrap for VS 2022 preview toolset.

• New: Add vc143, aka VS2022, aka cl.exe 17.x toolset support. Includes
  building engine and automatic detection of the prerelease toolset.
  -- Sergei Krivonos
• Allow alias targets to continue even if <build>no is in the usage
  requirement. Which allows composition of alias targets that may contain
  optional targets, like tests.
  -- Dmitry Arkhipov
• Fix use of JAMSHELL in gcc toolset.
  -- René Ferdinand Rivera Morell
• Fix compiling b2 enging such that it works when run in cross-architecture
  emulation context. I.e. when running arm binaries in QEMU 64 bit host.
  -- René Ferdinand Rivera Morell
• Default to 64bit MSVC on 64 bit hosts.
  -- Matt Chambers
• Remove /NOENTRY option for resource only DLLs to allow correct linking.
  -- gnaggnoyil
• Fix redefinition error of unix when compiling engine on OpenBSD.
  -- Brad Smith
• Fix building with clang on iOS and AppleTV having extra unrecognized
  compiler options.
  -- Konstantin Ivlev
• Add missing Boost.JSON to boost support module.
  -- Dmitry Arkhipov
• Add arm/arm64 target support in clang-win toolset.
  -- Volo Zyko
• Avoid warnings about threading model for qt5.
  -- psandana
• Unify Clang and GCC PCH creation.
  -- Nikita Kniazev
• Move Objective-C support to GCC toolset.
  -- Nikita Kniazev
• Support values for instruction-set feature for Xilinx ZYNQ.
  -- Thomas Brown
• MIPS: add generic mips architecture.
  -- YunQiang Su
• Fix preprocessing on MSVC compiler.
  -- Nikita Kniazev