[MacPorts] #31134: boost: reduce default threading variants

[MacPorts]

#31134: boost: reduce default threading variants
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 Reporter:  tvb377@…       |       Owner:  macports-tickets@…                   
     Type:  enhancement    |      Status:  new                                  
 Priority:  Normal         |   Milestone:                                       
Component:  ports          |     Version:  2.0.2                                
 Keywords:                 |        Port:  boost                                
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 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.

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Ticket URL: <https://trac.macports.org/ticket/31134>
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