Disable vs2019 on AZP.

B2 5.3.2

Fix backward incompatibility of `option.get` if an argument is not ad…

…ded with `args.add-arg`.

When such an argument isn't registered we fail to check `ARGV` directly as a backup, for backward compatibility. This fixes that problem by checking if an argument is registered, and if not falling back on the direct `ARGV` check.

fixes #440

Update release notes for 5.3.0.

Add release note for double msvc init fix.

Fix error ignore flag value for rmtree cleanup of temp test dirs.

Put back boost_mpi ref as we need until the usage is fixed.

PCH are broken on freebsd-gcc. Skip testing pch in freebsd.

Release notes.

Silence Apple clang warnings.