boost: reduce default threading variants

[tvb377@…]

This is a follow up to ticket #26466.

There is no difference (except in the library names) between threading=multi and threading=single on Darwin/OSX.

Linux (at least Suse) install the libs as threading=multi (default on linux) and layout=system (also default). boost_*-mt.so are simply links to the appropriate solibs.

Suggestion (partly already in #26466): Use layout=system, threading=multi (the defaults for darwin). If felt necessary, create symbolic links to -mt libs. Drop the variant no_single.

Benefit: reduce the horrible build times by 50%, be compatible with linux, save disk space.

Risk: presumably none (at least if providing the symbolic links).

I could prepare a patch or test some dependents if the idea is ok.

[adfernandes@…]

This will, I hope, be fixed with #31169.