4.4.0

Along with a variety of fixes this version introduces "dynamic" response file
support for some toolsets. This means that under most circumtances, if
supported by the toolset, response files are not generated. Instead the
command is expanded to include the options directly.

• New: Add response-file feature to control the kind of response file usage in
  toolset action.
  -- René Ferdinand Rivera Morell
• New: Add :O=value variable modifier for @() expansion.
  -- René Ferdinand Rivera Morell
• New: Add :<=value and :>=value variable modifiers for prefix and postfix
  values after the complete expansion of variable references.
  -- René Ferdinand Rivera Morell
• Remove one at time linking limit. Once upon a time this was a performance
  tweak as hardware and software was not up to doing multiple links at once.
  Common setups are better equipped.
  -- René Ferdinand Rivera Morell
• Fix building engine with GCC on AIX.
  -- René Ferdinand Rivera Morell
• Support building engine as either 32 or 64 bit addressing model.
  -- René Ferdinand Rivera Morell
• Basic support for building b2 engine on GNU/Hurd.
  -- Pino Toscano
• Update "borland" toolset to bcc32c for building B2.
  -- Tanzinul Islam
• Ensure Embarcadero toolset name is only "embtc".
  -- Tanzinul Islam
• Adapt for Emscripten 2.0 change of default behaviour for archives.
  -- Basil Fierz
• Fix path to bootstrap for back compat.
  -- René Ferdinand Rivera Morell
• Add missing BOOST_ROOT to boot strap search.
  -- René Ferdinand Rivera Morell
• Fix for engine compile on FreeBSD.
  -- René Ferdinand Rivera Morell
• Default MSVC to a native platform, and remove ambiguous implicit
  address-model ARM/ARM64 values.
  -- Nikita Kniazev
• Fix detection of MIPS32 for b2 engine build.
  -- Ivan Melnikov
• Enable building b2 engine with clang on Windows.
  -- Gei0r
• Fix building b2 engine with Intel Linux icpc.
  -- Alain Miniussi
• Reword build.sh to fix many bugs and to avoid use of common env vars.
  -- René Ferdinand Rivera Morell