4.8.0

This is mostly a bugfix release, but there is also now support for building with the new LoongArch architecture. This release also sees the movement of the b2 engine to using more C++ features to help in reducing memory leaks and cleaner exit semantics.

• New: Add support for LoongArch.
  -- Zhang Na
• Change engine build to use static Intel libs if available instead of C++
  runtime static libs to fix systems where the static C++ runtime is not
  available.
  -- Alain Miniussi
• Reorder msvc cflags and cxxflags, and add compileflags, to fix inability
  to override flags by users.
  -- Peter Dimov
• Don't quote RPATH on clang-linux to fix use of double-quotes to make it
  possible to use $ORIGIN.
  -- Dimitry Andric
• Fix b2 executable detection on kFreeBSD.
  -- Laurent Bigonville
• Add .ipp extension to header scanning and a valid C++ file.
  -- Jim King
• Fix missing install targets when build=no is in source target usage
  requirements.
  -- Dmitry Arkhipov
• Add some future versions of C++ to cxxstd feature.
  -- René Ferdinand Rivera Morell
• Fix many memory leaks in engine.
  -- René Ferdinand Rivera Morell
• Change abort/exit calls to clean exception handling to allow for memory
  cleanup in engine.
  -- René Ferdinand Rivera Morell