[MacPorts] #13274: update guide's X11 requirements (headers needed if libraries present)

[MacPorts]

#13274: update guide's X11 requirements (headers needed if libraries present)
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  Reporter:  afb at macports.org  |       Owner:  markd at macports.org     
      Type:  defect            |      Status:  closed                 
  Priority:  Normal            |   Milestone:  Website & Documentation
 Component:  guide             |     Version:  1.6.0                  
Resolution:  fixed             |    Keywords:                         
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Comment (by markd at macports.org):

 Also, I reordered the steps afterwards so installing X11 comes first.
 Someone on the list confirmed that X11SDK won't install without X11, which
 I didn't know or forgot.  But I left in the verbage telling users to
 manually select X11 (even though default on 10.5) and X11SDK (though it is
 installed automatically if X11 present) to keep it as compatible across OS
 version as possible and make it obvious that you need both.  Also, I
 didn't want to rely on automatic things to work right.

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Ticket URL: <http://trac.macosforge.org/projects/macports/ticket/13274#comment:6>
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