4.6.0

This release wraps up a few new features that make using some toolsets easier
(thanks to Nikita). It's now also possible to specify empty flags features on
the command line, like cxxfalgs=, and have those be ignored. This helps to
make CI scripts shorter as they don't need to handle those cases specially.
And as usual there are many bug fixes and adjustments. Thanks to everyone who
contributed to this release.

• New: Allow clang toolset to be auto-configured to a specific version by using
  toolset=clang-xx on the command line.
  -- Nikita Kniazev
• New: Include pch header automatically and on-demand on gcc and msvc toolset to
  mirror clang functionality.
  -- Nikita Kniazev
• New: Features that are narked as 'free' and 'optional' will now be ignored when
  the value specified on the command line is empty. Hence once can specify
  cxxflags= on the command line without errors.
  -- René Ferdinand Rivera Morell
• Preserve bootstrap.sh invoke arguments to forward to the build.sh script.
  -- tkoecker
• Remove use of local in buils.sh to be compatible with some, not fully
  capable, shells.
  -- Tanzinul Islam
• Workaround shell array ref error in build.sh on busybox shells.
  -- tkoecker
• Check for needing -pthread to build engine with gcc on some platforms.
  -- tkoecker
• Default to using clang on MacOS.
  -- Stéphan Kochen
• Add /python//numpy target to use as a dependency to communicate version
  specific properties.
  -- Peter Dimov
• Add default value for cxx and cxxflags from env vars CXX and CXXFLAGS
  when using the custom cxx toolset to build the engine.
  -- Samuel Debionne and René Ferdinand Rivera Morell
• Fix detection of intel-linux toolset installation when only the compiler
  executable is in the PATH.
  -- René Ferdinand Rivera Morell
• Fix b2 executable path determination for platforms that don't have a
  native method of getting the path to executables, like OpenBSD.
  -- René Ferdinand Rivera Morell
• Fix property.find error message.
  -- Thomas Brown