[Chicken-janitors] #179: add `platform` extension property

[Chicken Trac]

#179: add `platform` extension property
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 Reporter:  felix            |       Owner:       
     Type:  enhancement      |      Status:  new  
 Priority:  minor            |   Milestone:       
Component:  core tools       |     Version:  4.3.x
 Keywords:  chicken-install  |  
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 Some extensions are only suited for a particular set of platforms. A
 dedicated extension property, placed in the .meta file, could be used to
 declare extensions valid only for those platforms listed.

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Ticket URL: <http://www.irp.oist.jp/trac/chicken/ticket/179>
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