[MacPorts] #51706: platform statement: support an else clause

[MacPorts]

#51706: platform statement: support an else clause
-------------------------+--------------------------------
 Reporter:  rjvbertin@…  |      Owner:  macports-tickets@…
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  base         |    Version:
 Keywords:               |       Port:
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 As discussed on the ML a short while ago:

 This patch introduces support for an else clause in the `platform`
 statement. Typical use would a port that needs to distinguish between 2 OS
 versions that require mutually exclusive treatment in multiple locations.
 With an else clause means that can be done without duplicating platform
 statements and without resorting to if/else constructs with their less
 elegant syntax in the test expression.

 I haven't tested this yet but it might actually be possible to chain the
 commands:

 ```
 platform darwin 10 {
 # older
 } [else] platform darwin 11 {
 # old
 } else {
 # younger
 }
 ```

 If this is considered acceptable for a future MacPorts base version I'd
 really like to request a temporary vector to provide the new syntax ASAP,
 for instance through a PortGroup file.

-- 
Ticket URL: <https://trac.macports.org/ticket/51706>
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