List cxx11 requirements in Jamfile

[mloskot]

For example, mp11/test/Jamfile:

project :
  requirements [ requires cxx11_variadic_templates cxx11_template_aliases cxx11_decltype cxx11_hdr_tuple ] ;

Alternatively, as explained by @pdimov on cpplang.slack.com:

if you add cxxstd=11, they are satisfied, if you add cxxstd=03, they aren't
so if your test requires f.ex. decltype, it's run with cxxstd=11 and not run with cxxstd=03
but you can have tests that don't need decltype, which are run under cxxstd=03 as well, in principle

This will also help to skip the always failing builds in the regression waterfall ie. VS 2005, 2008, 2010, 2012, 2013:

[mloskot]

Asked via ML about Skipping regression builds for unsupported toolsets

[mloskot]

This has been fixed by #260