[MacPorts] #126: dependencies need to allow inclusion of variant

[MacPorts]

#126: dependencies need to allow inclusion of variant
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  Reporter:  jpm@…               |       Owner:  macports-tickets@…                   
      Type:  enhancement         |      Status:  new                                  
  Priority:  Normal              |   Milestone:  MacPorts Future                      
 Component:  base                |     Version:                                       
Resolution:                      |    Keywords:                                       
      Port:                      |  
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Comment(by ryandesign@…):

 The simplest would be to check whether a file you need is present or not.
 For example, if you need that port `foo` is installed with variant `+bar`,
 and variant `+bar` causes port `foo` to install the file
 `${prefix}/lib/libfoobar.dylib`, then declare a dependency on `port:foo`
 and in the `pre-fetch` phase check if the file
 `${prefix}/lib/libfoobar.dylib` exists, and if not, use `return -code
 error` to inform the user and exit.

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Ticket URL: <http://trac.macports.org/ticket/126#comment:24>
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