[MacPorts] #15712: Add versions to platforms

[MacPorts]

#15712: Add versions to platforms
--------------------------+----------------------------
  Reporter:  raimue       |      Owner:  larryv
      Type:  enhancement  |     Status:  assigned
  Priority:  Normal       |  Milestone:  MacPorts 2.6.0
 Component:  base         |    Version:
Resolution:               |   Keywords:
      Port:               |
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Comment (by mojca):

 I'm not aware of any case of one PortGroup specifying `supported_archs
 i386 x86_64` and another one specifying `supported_archs ppc i386` which
 would mean that the port would only support `i386`. This variable needs
 much less intervention in general, just occasionally specifying that the
 port could not be built 64-bit.

 @janstary: there's no need to do any whitelisting. For me it's enough to
 keep just blacklisting or specifying "broken" state in which case my
 proposal is basically equivalent to yours, just using a different keyword
 (blacklist instead of broken). I don't care which keyword exactly is used,
 but it's nice if it's clear that
 {{{
 whatever-keywoard {darwin >= 13}
 }}}
 doesn't mean that the port will unconditionally build on any given 10.9 or
 above since another expression in a PortGroup could additionally declare a
 broken support for another OS version.

-- 
Ticket URL: <https://trac.macports.org/ticket/15712#comment:40>
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